Thin film balun

ABSTRACT

A thin film balun that can be made smaller and thinner while maintaining required balun characteristics is provided. A thin film balun  1  includes: an unbalanced transmission line UL including a first coil portion C 1  and a second coil portion C 2 ; a balanced transmission line BL including a third coil portion C 3  and a fourth coil portion C 4  that are positioned facing and magnetically coupled to the first coil portion C 1  and the second coil portion C 2  respectively; an unbalanced terminal UT connected to the first coil portion C 1 ; a ground terminal G connected to the second coil portion C 2  via a C component D; and an electrode D 2  connected to the ground terminal G and facing a part of the second coil portion C 2 . The C component D is formed by the electrode D 2  and the part D 1  of the second coil portion C 2.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 14/031,655, filed Sep. 19, 2013, which is a divisional applicationof U.S. application Ser. No. 12/823,450, filed Jun. 25, 2010 and claimsthe benefit of Japanese Application Nos. 2009-156239 filed on Jun. 30,2009, No. 2009-157726 filed on Jul. 2, 2009, No. 2009-156241 filed onJun. 30, 2009, No. 2009-192593 filed on Aug. 21, 2009 and No.2009-192594 filed on Aug. 21, 2009, which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a balun (balun transformer) thatperforms conversion between unbalanced and balanced signals, and inparticular relates to a thin film balun that is formed by a thin filmprocess advantageous for smaller and thinner models.

2. Description of the Related Art

A wireless communication device includes various high frequency elementssuch as an antenna, a filter, an RF switch, a power amplifier, an RF-IC,and a balun. Of these elements, a resonant element such as an antenna ora filter handles (transmits) an unbalanced signal which is based on aground potential, whereas an RF-IC which generates or processes a highfrequency signal handles (transmits) a balanced signal. Accordingly,when electromagnetically connecting these two elements, a balun thatfunctions as an unbalanced-balanced converter is used.

Recently, there is a demand for smaller and thinner baluns for use inwireless LAN devices, mobile communication devices such as a mobilephone and a portable terminal, and so on. As one of such baluns, forexample, Japanese Patent Application Laid-Open No. 2006-270444 andJapanese Patent Application Laid-Open No. 2001-168607 each propose abalun having a structure in which a capacitor is connected to at leastone of an unbalanced circuit and a balanced circuit.

SUMMARY OF THE INVENTION

In view of this, it is desired to provide a thin film balun that can bemade smaller and thinner while maintaining required baluncharacteristics.

A thin film balun according to a first aspect of the present inventionis a thin film balun including: an unbalanced circuit; a balancedcircuit magnetically coupled to the unbalanced circuit; and a capacitorconnected to the unbalanced circuit, wherein a part of a line portionconstituting the unbalanced circuit serves as one electrode of thecapacitor.

According to this structure, the part of the line portion constitutingthe unbalanced circuit serves as one electrode of the capacitor, therebyintroducing a capacitance C in a resonant circuit of the thin filmbalun. This enables the thin film balun to be made smaller and thinner.As a result of conducting intense study, the present inventor has foundthat, when the part of the line portion serves as the electrode of thecapacitor, balance characteristics of a transmission signal can beadjusted while suppressing an increase in resonant frequency in passagecharacteristics of the transmission signal and maintaining the passagecharacteristics in a desired range (specifications).

Though details of a functional mechanism are still unclear, thefollowing assumption can be made. When the part of the line portionconstituting the unbalanced circuit serves as the electrode of thecapacitor, the capacitor is positioned near the unbalanced circuit.Therefore, not only the capacitance C is introduced directly, but also acapacitance like a stray capacitance is induced with its neighboringline portions, which leads to an increase in effective capacitance onthe unbalanced circuit side. Hence, by changing a position or an area ofthe line portion serving as the electrode of the capacitor, the balancecharacteristics can be adjusted easily while maintaining desired passagecharacteristics according to the specifications of the thin film balun.Note, however, that the function is not limited to such.

For example, the thin film balun according to the first aspect of thepresent invention is a thin film balun including: an unbalancedtransmission line; and a balanced transmission line positioned facingthe unbalanced transmission line and magnetically coupled to theunbalanced transmission line, wherein one end of the unbalancedtransmission line is connected to an unbalanced terminal, and an otherend of the unbalanced transmission line is connected to a groundterminal via a C component, and wherein the C component is formed by anopposite electrode and a part of a line portion constituting theunbalanced transmission line. As a more specific example, the thin filmbalun according to the present invention is a thin film balun including:an unbalanced transmission line including a first line portion and asecond line portion; a balanced transmission line including a third lineportion and a fourth line portion that are positioned facing the firstline portion and the second line portion and magnetically coupled to thefirst line portion and the second line portion, respectively; anunbalanced terminal connected to the first line portion; a groundterminal connected to the second line portion via a C component; and anopposite electrode connected to the ground terminal and facing a part ofthe second line portion, wherein the C component is formed by theopposite electrode and the part of the second line portion. According tothis structure, the same function as described above can be attained.

Preferably, the second line portion has an extension that extends from(that is formed by being extended from) a line portion of the secondline portion corresponding to the first line portion, wherein theopposite electrode faces the extension of the second line portion, andwherein the C component is formed by the opposite electrode and theextension of the second line portion. Here, the following assumption canbe made. According to this structure, the second line portion includesthe extension. Therefore, the magnetic coupling between the first lineportion and the third line portion and the magnetic coupling between thesecond line portion and the fourth line portion are maintainedequivalently. Moreover, since the C component is positioned outside aregion directly coupled to the balanced transmission line, thecapacitance is effectively introduced in the resonant circuit of thethin film balun, so that the thin film balun can be made smaller andthinner. As a result, the suppression of the resonant frequency increaseand the improvement of the balance characteristics of the transmissionsignal can both be achieved. Note, however, that the function is notlimited to such.

For example, the thin film balun according to the first aspect of thepresent invention is a thin film balun including: an unbalancedtransmission line including a first coil portion and a second coilportion; a balanced transmission line including a third coil portion anda fourth coil portion that are positioned facing the first coil portionand the second coil portion and magnetically coupled to the first coilportion and the second coil portion, respectively; an unbalancedterminal connected to the first coil portion; a ground terminalconnected to the second coil portion via a C component; and an oppositeelectrode connected to the ground terminal and facing a part of thesecond coil portion, wherein the C component is formed by the oppositeelectrode and the part of the second coil portion. According to thisstructure, the same function as described above can be attained.

For example, the opposite electrode faces a part of a coil conductorthat is closer to an outer peripheral coil conductor than an innerperipheral coil conductor in the second coil portion. Here, thefollowing assumption can be made. According to this structure, theopposite electrode can be appropriately separated from a coil openingthrough which a magnetic flux generated by the magnetic coupling of thesecond coil portion and the fourth coil portion passes. Therefore, asdescribed above, while maintaining the magnetic coupling between thefirst coil portion and the third coil portion and the magnetic couplingbetween the second coil portion and the fourth coil portion in anear-equivalent state, the capacitance is effectively introduced in theresonant circuit of the thin film balun, as a result of which thesuppression of the resonant frequency increase and the improvement ofthe balance characteristics of the transmission signal can both beachieved. Note, however, that the function is not limited to such. Inthis case, preferably, the opposite electrode faces a part of the outerperipheral coil conductor in the second coil portion.

Moreover, as a result of further conducting intense study, the presentinventor has found more favorable structural examples of the oppositeelectrode. As an example, it is useful that the opposite electrode isparallel to (i.e., provided along with) an extending direction of a partof a space between adjacent coil conductors of the second coil portion,and faces a part of the coil conductors on both sides of the space. Itis also preferable that the opposite electrode is parallel to (i.e.,provided along with) an extending direction of a part of a coilconductor of the second coil portion, and faces the part of the coilconductor.

According to the first aspect of the present invention, a part of theline portion constituting the unbalanced circuit serves as one electrodeof the capacitor connected to the unbalanced circuit, which enables thethin film balun to be made smaller and thinner while maintainingrequired balun characteristics.

A thin film balun according to a second aspect of the present inventionis a thin film balun including: an unbalanced transmission line; and abalanced transmission line positioned facing the unbalanced transmissionline and magnetically coupled to the unbalanced transmission line,wherein one end of the unbalanced transmission line is connected to anunbalanced terminal, and an other end of the unbalanced transmissionline is connected to a ground terminal via a C component, and whereinthe C component is formed by a first electrode and a second electrode,the first electrode being formed in a first layer that includes theunbalanced transmission line, and the second electrode being formed in asecond layer that faces the first layer and includes the balancedtransmission line.

According to this structure, the C component introduced in the resonantcircuit of the thin film balun is formed by the first electrode that isformed in the first layer including the unbalanced transmission line andthe second electrode that is formed in the second layer facing the firstlayer and including the balanced transmission line. Since there is noneed for a new layer for introducing the C component, the thin filmbalun can be made smaller and thinner. As a result of conducting intensestudy, the present inventor has found that, when the C component isprovided in this manner, the balance characteristics of the transmissionsignal can be adjusted while maintaining the passage characteristics ofthe transmission signal in the desired range (specifications).

Though details of a functional mechanism are still unclear, thefollowing assumption can be made. Since the C component is positionednear the balanced-unbalanced circuit, not only the capacitance C isintroduced directly, but also a capacitance like a stray capacitance isinduced with its neighboring line portions, which leads to an increasein effective capacitance on the unbalanced circuit side. This enablesthe thin film balun to be made smaller and thinner while maintainingdesired passage characteristics of the thin film balun. Note, however,that the function is not limited to such.

Furthermore, since there is no need for a new layer for introducing theC component, this structure of the thin film balun according to thepresent invention also leads to a reduction in the number ofmanufacturing process steps, which contributes to improved productivityand simplified process control.

For example, the thin film balun according to the second aspect of thepresent invention is a thin film balun including: an unbalancedtransmission line including a first line portion and a second lineportion; a balanced transmission line including a third line portion anda fourth line portion that are positioned facing the first line portionand the second line portion and magnetically coupled to the first lineportion and the second line portion, respectively; an unbalancedterminal connected to the first line portion; and a ground terminalconnected to the second line portion via a C component, wherein the Ccomponent is provided between a first layer that includes the unbalancedtransmission line and a second layer that faces the first layer via adielectric layer and includes the balanced transmission line. Accordingto this structure, the same function as described above can be attained.

Preferably, the first layer includes a first electrode that extends fromthe second line portion of the unbalanced transmission line, wherein thesecond layer includes a second electrode that is connected to the groundterminal and positioned facing the first electrode via the dielectriclayer, and wherein the C component is formed by the first electrode andthe second electrode. Here, the following assumption can be made.According to this structure, the first electrode extends from the secondline portion, thereby allowing the second line portion to be equal inlength to the first line portion. Therefore, while equivalentlymaintaining the magnetic coupling between the first line portion and thethird line portion and the magnetic coupling between the second lineportion and the fourth line portion, the capacitance is effectivelyintroduced in the resonant circuit of the thin film balun. As a result,the suppression of the resonant frequency increase and the improvementof the balance characteristics of the transmission signal can both beachieved. Note, however, that the function is not limited to such.

For example, the thin film balun according to the second aspect of thepresent invention is a thin film balun including: an unbalancedtransmission line including a first coil portion and a second coilportion; a balanced transmission line including a third coil portion anda fourth coil portion that are positioned facing the first coil portionand the second coil portion and magnetically coupled to the first coilportion and the second coil portion, respectively; an unbalancedterminal connected to the first coil portion; and a ground terminalconnected to the second coil portion via a C component, wherein the Ccomponent is provided between a first layer that includes the unbalancedtransmission line and a second layer that faces the first layer via adielectric layer and includes the balanced transmission line. Accordingto this structure, the same function as described above can be attained.

Preferably, the first layer includes a first electrode that extends froman outer peripheral end of the second coil portion of the unbalancedtransmission line, wherein the second layer includes a second electrodethat is connected to the ground terminal and positioned facing the firstelectrode via the dielectric layer, and wherein the C component isformed by the first electrode and the second electrode. Here, thefollowing assumption can be made. According to this structure, since theC component is positioned near the balanced-unbalanced circuit, not onlythe capacitance C is introduced directly, but also a capacitance like astray capacitance is induced with its neighboring line portions, whichleads to an increase in effective capacitance on the unbalanced circuitside. Hence, the thin film balun can be made smaller and thinner whilemaintaining desired passage characteristics of the thin film balun. Inaddition, according to this structure, the first electrode can beappropriately separated from a coil opening through which a magneticflux generated by the magnetic coupling of the second coil portion andthe fourth coil portion passes. Therefore, as described above, whilemaintaining the magnetic coupling between the first coil portion and thethird coil portion and the magnetic coupling between the second coilportion and the fourth coil portion in a near-equivalent state, thecapacitance is effectively introduced in the resonant circuit of thethin film balun. Note, however, that the function is not limited tosuch.

According to the second aspect of the present invention, the C componentconnected to the second line portion is provided between the first layerincluding the unbalanced transmission line and the second layerincluding the balanced transmission line, which enables the thin filmbalun to be further made smaller and thinner.

A thin film balun according to a third aspect of the present inventionis a thin film balun including: an unbalanced transmission line; abalanced transmission line facing the unbalanced transmission line andelectromagnetically coupled to the unbalanced transmission line; acapacitor electrode facing a part of the unbalanced transmission linethat faces the balanced transmission line, to form a capacitor; and anunbalanced terminal connected to the unbalanced transmission line andthe capacitor electrode.

According to this structure, the capacitor is introduced in the resonantcircuit of the thin film balun, so that the resonant frequency of thethin film balun is shifted toward lower frequencies. This suppresses anincrease in resonant frequency caused by miniaturization of the thinfilm balun. It has also been found that the balance characteristics ofthe thin film balun can be improved by providing the capacitor so thatthe capacitor electrode faces the part of the unbalanced transmissionline facing the balanced transmission line.

Though details of a functional mechanism are still unclear, thefollowing assumption can be made. The part of the unbalancedtransmission line facing the balanced transmission line in the capacitoris a dominant part of electromagnetic coupling with the balancedtransmission line. This being so, by forming the capacitor in such amanner that the capacitor electrode faces this part, a state of couplingof the unbalanced transmission line with the balanced transmission linecan be changed significantly. That is, by introducing the capacitor(capacitance) in a predetermined position of the unbalanced transmissionline, the part of the unbalanced transmission line that contributes tothe coupling with the balanced transmission line can be changed.Moreover, by introducing the capacitor in this way, characteristicimpedance of the entire transmission line can be changed. The change ofthe part contributing to the coupling and the change of thecharacteristic impedance allow the balance characteristics of the thinfilm balun to be improved favorably. Note, however, that the function isnot limited to such.

The structure according to the third aspect of the present inventionalso includes the case where another capacitor electrode connected tothe unbalanced transmission line is provided between the part of theunbalanced transmission line and the capacitor electrode facing the partof the unbalanced transmission line. Here, it is preferable that thepart of the unbalanced transmission line serves as another capacitorelectrode. According to this structure, there is no need for a new layerfor providing another capacitor electrode, which leads to a reduction inthe number of manufacturing process steps. This contributes to improvedproductivity and simplified process control.

Moreover, the thin film balun according to the third aspect of thepresent invention is a thin film balun including: an unbalancedtransmission line including a first line portion and a second lineportion; a balanced transmission line including a third line portion anda fourth line portion that are positioned facing the first line portionand the second line portion and electromagnetically coupled to the firstline portion and the second line portion, respectively; a capacitorelectrode facing a part of the second line portion that faces the fourthline portion, to form a capacitor; and an unbalanced terminal connectedto the first line portion and the capacitor electrode. According to thisstructure, the same function as described above can be attained.

Preferably, the thin film balun further includes a capacitor electrodefacing a part of the first line portion that faces the third lineportion, to form a capacitor, wherein the unbalanced terminal isconnected to the first line portion, the capacitor electrode facing thepart of the second line portion, and the capacitor electrode facing thepart of the first line portion. Here, the following assumption can bemade. According to this structure, the part contributing to the couplingbetween the third line portion and the first line portion can be changedin addition to the coupling between the fourth line portion and thesecond line portion. This allows the balance characteristics of the thinfilm balun to be further improved because the two couplings can beadjusted. Note, however, that the function is not limited to such.

Furthermore, the thin film balun according to the third aspect of thepresent invention is a thin film balun including: an unbalancedterminal; an unbalanced circuit, one end of the unbalanced circuit beingconnected to the unbalanced terminal and an other end of the unbalancedcircuit being an open end; and a balanced circuit electromagneticallycoupled to the unbalanced circuit, wherein a capacitor is connected to apart of the unbalanced circuit except the open end, and also connectedto the unbalanced terminal.

Here, the following assumption can be made. According to this structure,the capacitor is introduced in the resonant circuit of the thin filmbalun, so that the resonant frequency of the thin film balun is shiftedtoward lower frequencies. This suppresses an increase in resonantfrequency caused by miniaturization of the thin film balun. Moreover, aninner region than the open end of the unbalanced transmission line is apart where the electromagnetic coupling with the balanced transmissionline is relatively strong as compared with the open end. By forming thecapacitor in such a manner that the capacitor electrode faces this part,the part of the unbalanced transmission line that contributes to thecoupling with the balanced transmission line can be changed. Moreover,by introducing the capacitor, the characteristic impedance of the entiretransmission line can be changed. The change of the part contributing tothe coupling and the change of the characteristic impedance allow thebalance characteristics of the thin film balun to be improved. Note,however, that the function is not limited to such.

According to the third aspect of the present invention, by introducingthe capacitor between the unbalanced terminal and the part of theunbalanced transmission line facing the balanced transmission line, thebalance characteristics of the thin film balun can be improved whilesuppressing the frequency increase of the thin film balun caused byminiaturization. This enables the thin film balun to be further madesmaller and thinner.

A thin film balun according to a fourth aspect of the present inventionis a thin film balun including: an unbalanced transmission lineincluding a first line portion and a second line portion; a balancedtransmission line including a third line portion and a fourth lineportion that are positioned facing the first line portion and the secondline portion and magnetically coupled to the first line portion and thesecond line portion, respectively; an unbalanced terminal connected tothe first line portion; a ground terminal connected to the first lineportion via a C component; and an opposite electrode connected to theground terminal and facing a part of the first line portion, wherein theC component is formed by the opposite electrode and the part of thefirst line portion.

According to this structure, the C component introduced in the resonantcircuit of the thin film balun is formed using the part of the firstline portion constituting the unbalanced transmission line. This makesit unnecessary to add a new layer for introducing the C component, sothat the thin film balun can be made smaller and thinner. As a result ofconducting intense study, the present inventor has found that, when theC component is provided in this manner, the balance characteristics ofthe transmission signal can be adjusted while improving the passagecharacteristics of the transmission signal.

Though details of a functional mechanism are still unclear, thefollowing assumption can be made. When the part of the first lineportion constituting the unbalanced transmission line serves as theelectrode of the capacitor, the capacitor is positioned near theunbalanced transmission line. Therefore, not only the capacitance C isintroduced directly, but also a capacitance like a stray capacitance isinduced with its neighboring line portions, which influences thecharacteristic impedance. Note, however, that the function is notlimited to such.

As a result of further conducting intense study, the present inventorhas found more favorable structural examples of the opposite electrode.As an example, it has been found useful that any of the second lineportion and the fourth line portion has a larger width than acorresponding one of the first line portion and the third line portion.

For example, the thin film balun according to the fourth aspect of thepresent invention is a thin film balun including: an unbalancedtransmission line including a first coil portion and a second coilportion; a balanced transmission line including a third coil portion anda fourth coil portion that are positioned facing the first coil portionand the second coil portion and magnetically coupled to the first coilportion and the second coil portion, respectively; an unbalancedterminal connected to the first coil portion; a ground terminalconnected to the first coil portion via a C component; and an oppositeelectrode connected to the ground terminal and facing a part of thefirst coil portion, wherein the C component is formed by the oppositeelectrode and the part of the first coil portion. According to thisstructure, the same function as described above can be attained.

Preferably, the opposite electrode faces a part of a coil conductor thatis closer to an outer peripheral coil conductor than an inner peripheralcoil conductor in the first coil portion. According to this structure,the opposite electrode can be appropriately separated from a coilopening through which a magnetic flux generated by the magnetic couplingof the second coil portion and the fourth coil portion passes. Hence,while maintaining the magnetic coupling between the first coil portionand the third coil portion and the magnetic coupling between the secondcoil portion and the fourth coil portion in a near-equivalent state, thecapacitance is effectively introduced in the resonant circuit of thethin film balun. This makes it possible to adjust the balancecharacteristics while maintaining high passage characteristics of thetransmission signal. Note, however, that the function is not limited tosuch.

According to the fourth aspect of the present invention, the part of thefirst line portion that constitutes the unbalanced transmission line andis connected to the unbalanced terminal serves as one electrode of thecapacitor. This has a significant effect of making the thin film balunsmaller and thinner, and also achieves balance characteristic adjustmentand insertion loss reduction.

A thin film balun according to a fifth aspect of the present inventionis a thin film balun including: an unbalanced transmission line; abalanced transmission line facing the unbalanced transmission line andelectromagnetically coupled to the unbalanced transmission line; and acapacitor electrode facing a part of the balanced transmission line toform a capacitor, and connected to a ground terminal.

Here, the following assumption can be made. According to this structure,a state of coupling of the balanced transmission line with theunbalanced transmission line can be significantly changed by thecapacitor electrode facing the part of the balanced transmission line.That is, by introducing the capacitor (capacitance) in a predeterminedposition of the balanced transmission line, the part of the balancedtransmission line that contributes to the coupling with the unbalancedtransmission line can be changed. Moreover, by introducing the capacitorin this way, the characteristic impedance of the entire transmissionline can be changed. The change of the part contributing to the couplingand the change of the characteristic impedance allow the passagecharacteristics of the thin film balun to be improved favorably. Note,however, that the function is not limited to such.

This structure according to the present invention also includes the casewhere another capacitor electrode connected to the balanced transmissionline is provided between the part of the balanced transmission line andthe capacitor electrode facing the part of the balanced transmissionline. Here, it is preferable that the part of the balanced transmissionline serves as another capacitor electrode. According to this structure,there is no need for a new layer for providing another capacitorelectrode, which leads to a reduction in the number of manufacturingprocess steps. This contributes to improved productivity and simplifiedprocess control.

Moreover, the thin film balun according to the fifth aspect of thepresent invention is a thin film balun including: an unbalancedtransmission line including a first line portion and a second lineportion; a balanced transmission line including a third line portion anda fourth line portion that are positioned facing the first line portionand the second line portion and electromagnetically coupled to the firstline portion and the second line portion, respectively; and a capacitorelectrode facing a part of at least one of the third line portion andthe fourth line portion to form a capacitor, and connected to a groundterminal. According to this structure, the same function as describedabove can be attained.

Preferably, a connector electrically connecting the third line portionand the fourth line portion and the capacitor electrode are formed in afirst layer, wherein the third line portion and the fourth line portionof the balanced transmission line are formed in a second layer, andwherein the first line portion and the second line portion of theunbalanced transmission line are formed in a third layer. By forming thecapacitor electrode in the same layer as the connector, there is no needfor a new layer for providing another capacitor electrode. Therefore,the thin film balun can be made smaller and thinner. This also leads toa reduction in the number of manufacturing process steps, therebycontributing to improved productivity and simplified process control.

Furthermore, the thin film balun according to the fifth aspect of thepresent invention is a thin film balun including: an unbalanced circuit;a balanced circuit electromagnetically coupled to the unbalancedcircuit; and a ground terminal, wherein a capacitor is connected to apart of the balanced circuit, and also connected to the ground terminal.

Here, the following assumption can be made. According to this structure,by connecting the capacitor to the part of the balanced circuit, a stateof coupling of the balanced circuit with the unbalanced circuit can bechanged significantly. That is, by introducing the capacitor(capacitance) in a predetermined position of the balanced circuit, thepart of the balanced circuit that contributes to the coupling with theunbalanced circuit can be changed. Furthermore, by introducing thecapacitor in this way, the characteristic impedance of the entirecircuit can be changed. The change of the part contributing to thecoupling and the change of the characteristic impedance allow thepassage characteristics of the thin film balun to be improved favorably.Note, however, that the function is not limited to such.

According to the fifth aspect of the present invention, the capacitor isintroduced between the part of the balanced transmission line and theground terminal, with it being possible to achieve excellent passagecharacteristics for the thin film balun which is made smaller andthinner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit diagram showing a structure of a thinfilm balun 1 in a first embodiment of the present invention.

FIG. 2 is a vertical sectional view showing the structure of the thinfilm balun 1.

FIG. 3 is a horizontal sectional view showing a wiring layer M0 of thethin film balun 1.

FIG. 4 is a horizontal sectional view showing a wiring layer M1 of thethin film balun 1.

FIG. 5 is a horizontal sectional view showing a wiring layer M2 of thethin film balun 1.

FIG. 6 is a horizontal sectional view showing a wiring layer M3 of thethin film balun 1.

FIG. 7 is a horizontal sectional view showing the wiring layer M0 of athin film balun 1A.

FIG. 8 is a horizontal sectional view showing the wiring layer M0 of athin film balun 1B.

FIG. 9 is a horizontal sectional view showing the wiring layer M0 of athin film balun 1C.

FIG. 10 is a horizontal sectional view showing the wiring layer M0 of athin film balun 1D.

FIG. 11 is a graph showing passage characteristic evaluation results.

FIG. 12 is a graph showing phase difference evaluation results.

FIG. 13 is a graph showing amplitude difference evaluation results.

FIG. 14 is a horizontal sectional view showing the wiring layer M0 of athin film balun 2 in the first embodiment of the present invention.

FIG. 15 is a horizontal sectional view showing the wiring layer M1 ofthe thin film balun 2.

FIG. 16 is a horizontal sectional view showing the wiring layer M0 of athin film balun 2A.

FIG. 17 is a horizontal sectional view showing the wiring layer M0 of athin film balun 2B.

FIG. 18 is a horizontal sectional view showing the wiring layer M0 of athin film balun 2C.

FIG. 19 is a horizontal sectional view showing the wiring layer M0 of athin film balun 2D.

FIG. 20 is a graph showing passage characteristic evaluation results.

FIG. 21 is a graph showing phase difference evaluation results.

FIG. 22 is a graph showing amplitude difference evaluation results.

FIG. 23 is a horizontal sectional view showing the wiring layer M0 ofthin film baluns 1E and 1F.

FIG. 24 is a horizontal sectional view showing the wiring layer M1 ofthe thin film baluns 2, 2A, and 2B.

FIG. 25 is a graph showing passage characteristic evaluation results.

FIG. 26 is a graph showing phase difference evaluation results.

FIG. 27 is a graph showing amplitude difference evaluation results.

FIG. 28 is a horizontal sectional view showing a wiring layer B1 of athin film balun of a comparative example.

FIG. 29 is a graph showing passage characteristic evaluation results ofthe thin film balun of the comparative example.

FIG. 30 is an equivalent circuit diagram showing a structure of a thinfilm balun 3 in a second embodiment of the present invention.

FIG. 31 is a vertical sectional view showing the structure of the thinfilm balun 3.

FIG. 32 is a horizontal sectional view showing the wiring layer M1 ofthe thin film balun 3.

FIG. 33 is a horizontal sectional view showing the wiring layer M2 ofthe thin film balun 3.

FIG. 34 is a horizontal sectional view showing the wiring layer M3 ofthe thin film balun 3.

FIG. 35 is a horizontal sectional view showing the wiring layer B1 of athin film balun 3A of a comparative example.

FIG. 36 is a horizontal sectional view showing the wiring layer M0 ofthe thin film balun 3A.

FIG. 37 is a horizontal sectional view showing the wiring layer M1 ofthe thin film balun 3A.

FIG. 38 is a horizontal sectional view showing the wiring layer M2 ofthe thin film balun 3A.

FIG. 39 is a horizontal sectional view showing the wiring layer B1 of athin film balun 3B of a comparative example.

FIG. 40 is a horizontal sectional view showing the wiring layer M0 ofthe thin film balun 3B.

FIG. 41 is a graph showing passage characteristic evaluation results.

FIG. 42 is a graph showing phase difference evaluation results.

FIG. 43 is a graph showing amplitude difference evaluation results.

FIG. 44 is a horizontal sectional view showing the wiring layer B1 of athin film balun of a comparative example.

FIG. 45 is a graph showing passage characteristic evaluation results ofthe thin film balun of the comparative example.

FIG. 46 is a vertical sectional view showing a structure of a thin filmbalun 3C in the second embodiment of the present invention.

FIG. 47 is an equivalent circuit diagram showing a structure of a thinfilm balun 4 in a third embodiment of the present invention.

FIG. 48 is a vertical sectional view showing the structure of the thinfilm balun 4.

FIG. 49 is a horizontal sectional view showing the wiring layer M0 of athin film balun 4A.

FIG. 50 is a horizontal sectional view showing the wiring layer M1 ofthe thin film balun 4A.

FIG. 51 is a horizontal sectional view showing the wiring layer M2 ofthe thin film balun 4A.

FIG. 52 is a horizontal sectional view showing the wiring layer M3 ofthe thin film balun 4A.

FIG. 53 is a horizontal sectional view showing the wiring layer M0 of athin film balun 4B.

FIG. 54 is a graph showing passage characteristic evaluation results.

FIG. 55 is a horizontal sectional view showing the wiring layer M0 of athin film balun 4C.

FIG. 56 is a horizontal sectional view showing the wiring layer M0 of athin film balun 4D.

FIG. 57 is a horizontal sectional view showing the wiring layer M0 of athin film balun 4E.

FIG. 58 is a horizontal sectional view showing the wiring layer M0 of athin film balun 4F.

FIG. 59 is a graph showing passage characteristic evaluation results.

FIG. 60 is a graph showing phase difference evaluation results.

FIG. 61 is a graph showing amplitude difference evaluation results.

FIG. 62 is an equivalent circuit diagram showing a structure of a thinfilm balun 5 in the third embodiment of the present invention.

FIG. 63 is a horizontal sectional view showing the wiring layer M0 of athin film balun 5A.

FIG. 64 is a horizontal sectional view showing the wiring layer M0 of athin film balun 5B.

FIG. 65 is a graph showing passage characteristic evaluation results.

FIG. 66 is a graph showing phase difference evaluation results.

FIG. 67 is a graph showing amplitude difference evaluation results.

FIG. 68 is an equivalent circuit diagram showing a structure of a thinfilm balun 6 in a fourth embodiment of the present invention.

FIG. 69 is a vertical sectional view showing the structure of the thinfilm balun 6.

FIG. 70 is a horizontal sectional view showing the wiring layer M0 ofthe thin film balun 6.

FIG. 71 is a horizontal sectional view showing the wiring layer M1 ofthe thin film balun 6.

FIG. 72 is a horizontal sectional view showing the wiring layer M2 ofthe thin film balun 6.

FIG. 73 is a horizontal sectional view showing the wiring layer M3 ofthe thin film balun 6.

FIG. 74 is a horizontal sectional view showing the wiring layer M0 of athin film balun 6A.

FIG. 75 is a horizontal sectional view showing the wiring layer M0 of athin film balun 6B.

FIG. 76 is a graph showing passage characteristic evaluation results.

FIG. 77 is a graph showing phase difference evaluation results.

FIG. 78 is a graph showing amplitude difference evaluation results.

FIG. 79 is an equivalent circuit diagram showing a structure of a thinfilm balun 7 in a fifth embodiment of the present invention.

FIG. 80 is a vertical sectional view showing the structure of the thinfilm balun 7.

FIG. 81 is a horizontal sectional view showing the wiring layer M0 of athin film balun 7A.

FIG. 82 is a horizontal sectional view showing the wiring layer M1 ofthe thin film balun 7A.

FIG. 83 is a horizontal sectional view showing the wiring layer M2 ofthe thin film balun 7A.

FIG. 84 is a horizontal sectional view showing the wiring layer M0 of athin film balun 7B.

FIG. 85 is an equivalent circuit diagram showing a structure of a thinfilm balun 7R of a comparative example.

FIG. 86 is a horizontal sectional view showing the wiring layer M0 ofthe thin film balun 7R.

FIG. 87 is a horizontal sectional view showing the wiring layer M1 ofthe thin film balun 7R.

FIG. 88 is a graph showing passage characteristic evaluation results.

FIG. 89 is a graph showing phase difference evaluation results.

FIG. 90 is a graph showing amplitude difference evaluation results.

FIG. 91 is a horizontal sectional view showing the wiring layer M0 of athin film balun 7C.

FIG. 92 is a graph showing passage characteristic evaluation results.

FIG. 93 is a graph showing phase difference evaluation results.

FIG. 94 is a graph showing amplitude difference evaluation results.

FIG. 95 is a horizontal sectional view showing the wiring layer M0 of athin film balun 7D.

FIG. 96 is a horizontal sectional view showing the wiring layer M0 of athin film balun 7E.

FIG. 97 is a graph showing passage characteristic evaluation results.

FIG. 98 is a graph showing phase difference evaluation results.

FIG. 99 is a graph showing amplitude difference evaluation results.

FIG. 100 is an equivalent circuit diagram showing a structure of a thinfilm balun 8 in the fifth embodiment of the present invention.

FIG. 101 is a horizontal sectional view showing the wiring layer M0 of athin film balun 8A.

FIG. 102 is a horizontal sectional view showing the wiring layer M0 of athin film balun 8B.

FIG. 103 is an equivalent circuit diagram showing a structure of a thinfilm balun 8R of a comparative example.

FIG. 104 is a horizontal sectional view showing the wiring layer M0 ofthe thin film balun 8R.

FIG. 105 is a horizontal sectional view showing the wiring layer M1 ofthe thin film balun 8R.

FIG. 106 is a graph showing passage characteristic evaluation results.

FIG. 107 is a graph showing phase difference evaluation results.

FIG. 108 is a graph showing amplitude difference evaluation results.

FIG. 109 is an equivalent circuit diagram showing a structure of a thinfilm balun 9 in the fifth embodiment of the present invention.

FIG. 110 is a horizontal sectional view showing the wiring layer M0 of athin film balun 9A.

FIG. 111 is a graph showing passage characteristic evaluation results.

FIG. 112 is a graph showing phase difference evaluation results.

FIG. 113 is a graph showing amplitude difference evaluation results.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes embodiments of the present invention withreference to drawings. Note that the same components in the drawings aregiven the same reference signs, and repeated description is omitted.Moreover, the positional relationships such as top, bottom, left, andright are based on the positional relationships shown in the drawings,unless otherwise specified. Furthermore, scale ratios of the drawingsare not limited to the illustrated ratios. Note also that the followingembodiments are merely examples for describing the present invention,and the present invention is not limited to the embodiments. Variouschanges can be made without departing from the scope of the presentinvention.

First Embodiment

A thin film balun in a first embodiment corresponding to the firstaspect of the present invention is described first. FIG. 1 is anequivalent circuit diagram showing a structure of the thin film balun inthe first embodiment of the present invention. A thin film balun 1includes an unbalanced transmission line (unbalanced circuit) UL inwhich a line portion L1 (first line portion) and a line portion L2(second line portion) are connected in series, and a balancedtransmission line (balanced circuit) BL in which a line portion L3(third line portion) and a line portion L4 (fourth line portion) areconnected in series. The line portions L1 and L3 form magnetic coupling,and the line portions L2 and L4 form magnetic coupling.

In the thin film balun 1, an end of the line portion L1 other than anend connected to the line portion L2 is connected to an unbalancedterminal UT, and a part of an end of the line portion L2 other than anend connected to the line portion L1 is connected to a ground terminal G(ground potential) via a capacitor D which is a C component (capacitancecomponent). The capacitor D is formed by an electrode D1 composed of thepart of the other end of the line portion L2 and an electrode D2(opposite electrode) connected to the ground terminal G where theelectrodes D1 and D2 face each other via an appropriate dielectric. Onthe other hand, an end of the line portion L3 other than an endconnected to the line portion L4 is connected to a balanced terminalBT1, and an end of the line portion L4 other than an end connected tothe line portion L3 is connected to a balanced terminal BT2. Moreover,the connected parts of the line portions L3 and L4 are grounded to thesame potential as the ground terminal G.

Lengths of the above-mentioned line portions L1 to L4 vary depending onspecifications of the thin film balun 1. For example, the lengths may beset so as to form a quarter-wavelength (λ/4) resonator circuit of atransmission signal which is subject to conversion. Moreover, the lineportions L1 to L4 may be arbitrarily shaped so long as theabove-mentioned magnetic coupling is formed. Example shapes include aspiral (coil form), a zigzag, a straight line, and a curved line.

A basic operation of the thin film balun 1 is described below, withreference to FIG. 1. In the thin film balun 1, when an unbalanced signalis input to the unbalanced terminal UT, the unbalanced signal propagatesthrough the line portions L1 and L2. By the magnetic coupling (firstmagnetic coupling) of the line portions L1 and L3 and the magneticcoupling (second magnetic coupling) of the line portions L2 and L4, theinput unbalanced signal is converted to two balanced signals that differin phase by 180° (π), and the two balanced signals are output from thebalanced terminals BT1 and BT2. A converting operation from balancedsignals to an unbalanced signal is the reverse of the above-mentionedconverting operation from an unbalanced signal to balanced signals.

A wiring structure of the thin film balun is described below. FIG. 2 isa vertical sectional view schematically showing the wiring structure ofthe thin film balun 1. As shown in FIG. 2, wiring layers M0, M1, M2, andM3 are formed in this order on an insulating substrate 100 of alumina orthe like. For instance, the unbalanced transmission line UL mentionedabove is formed by the wiring layer M1, and the balanced transmissionline BL is formed by the wiring layer M2. Insulating layers are formedbetween wires in the same wiring layer and between different wiringlayers. For example, an insulating layer 102 made of silicon nitride isformed between the wiring layers M0 and M1. Polyimide or the like isused as an insulating layer 101 in other parts. Note that the materialsare not limited to the above, and not only inorganic insulators such assilicon nitride, alumina, and silica but also organic insulators such aspolyimide and epoxy resin may be selected according to need. Theunbalanced terminal UT, the balanced terminals BT1 and BT2, and theground terminal G are formed through all insulating layers. Thus, thethin film balun 1 is composed of a thin film multilayer structure formedon the insulating substrate 100.

A pattern of each of the wiring layers M0, M1, M2, and M3 of the thinfilm balun in the first embodiment is described in detail below. Coilportions are used as the line portions L1 to L4 in each of the followingexamples 1 and 2.

Example 1

FIGS. 3 to 6 are each a horizontal sectional view schematically showinga different one of the wiring layers of the thin film balun 1 of theexample 1 in the first embodiment of the present invention. As shown inFIGS. 3 to 6, the unbalanced terminal UT, the balanced terminals BT1 andBT2, and the ground terminal G are formed in all of the wiring layers M0to M3. Each of the terminals UT, BT1, BT2, and G is electricallyconnected between different layers via a through hole P. Note that allthrough holes P shown in FIGS. 3 to 6 are plated with a metal conductorfor electrical conduction of upper and lower layers. A structure of eachwiring layer is described in detail below.

As shown in FIG. 3, the electrode D2 of the capacitor D is formed in thewiring layer M0 on the insulating substrate 100, at a position facing apart of a coil portion C2 in the wiring layer M1. The electrode D2 isconnected to the ground terminal G. In the first embodiment, theelectrode D2 of the capacitor D is parallel to a coil conductor that isan outer peripheral coil conductor in the coil portion C2 and extendsfrom the ground terminal G toward the balanced terminal BT2.

As shown in FIG. 4, a coil portion C1 (first coil portion, first lineportion) and the coil portion C2 (second coil portion, second lineportion) that constitute the unbalanced transmission line UL are formedadjacent to each other in the wiring layer M1. Each of the coil portionsC1 and C2 forms an equivalent of a quarter-wavelength (λ/4) resonator.An outer end 11 a of a coil conductor 11 constituting the coil portionC1 is connected to the unbalanced terminal UT, and an inner end 11 b ofthe coil conductor 11 is connected to a through hole P. An inner end 12b of a coil conductor 12 constituting the coil portion C2 is connectedto a through hole P. An outer end 12 a of the coil conductor 12 is open,but a part of the outer peripheral coil conductor 12 serves as theelectrode D1 of the capacitor D and faces the electrode D2 in the wiringlayer M0.

As shown in FIG. 5, a coil portion C3 (third coil portion, third lineportion) and a coil portion C4 (fourth coil portion, fourth lineportion) that constitute the balanced transmission line BL are formedadjacent to each other in the wiring layer M2. Each of the coil portionsC3 and C4 forms an equivalent of a quarter-wavelength (λ/4) resonator,as with the coil portions C1 and C2. The coil portions C3 and C4 of thebalanced transmission line BL are positioned facing the coil portions C1and C2 of the unbalanced transmission line UL respectively, and thefacing portions are magnetically coupled to form couplers. An outer end21 a of a coil conductor 21 constituting the coil portion C3 isconnected to the balanced terminal BT1, and an inner end 21 b of thecoil conductor 21 is connected to a through hole P. An outer end 22 a ofa coil conductor 22 constituting the coil portion C4 is connected to thebalanced terminal BT2, and an inner end 22 b of the coil conductor 22 isconnected to a through hole P.

As shown in FIG. 6, a wire 31 for connecting the coil portions C3 and C4to the ground terminal G and a wire 32 for connecting the coil portionsC1 and C2 are formed in the wiring layer M3. The wire 31 is a branchwire formed so as to connect two through holes P to the ground terminalG. The wire 31 is connected to the end 21 b of the coil conductor 21 andthe end 22 b of the coil conductor 22 formed in the wiring layer M2, viathe two through holes P. The wire 32 is connected to the end 11 b of thecoil conductor 11 and the end 12 b of the coil conductor 12 formed inthe wiring layer M1, via through holes P.

Thus, in the example 1, the thin film balun 1 forming the equivalentcircuit shown in FIG. 1 is obtained by a multilayer wiring structure inwhich the two coil portions C1 and C2 constituting the unbalancedtransmission line are formed in the wiring layer M1 which is one layer,the two coil portions C3 and C4 constituting the balanced transmissionline are formed in the wiring layer M2 which is another layer adjacentto the wiring layer M1, the wire 32 connecting the coil portions C1 andC2 and the wire 31 connecting the coil portions C3 and C4 are formed inthe wiring layer M3 which is another layer adjacent to the wiring layerM2 on an opposite side to the wiring layer M1, and the electrode D2facing the part of the coil conductor 12 in the coil portion C2 to formthe capacitor D is formed in the wiring layer M0 which is another layeradjacent to the wiring layer M1 on an opposite side to the wiring layerM2.

Example 1A

FIG. 7 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 1A of an example 1A. Structures other thanthe wiring layer M0 are the same as those in the example 1. In the thinfilm balun 1A shown in FIG. 7, the electrode D2 of the capacitor D ispositioned so as to be parallel to an extending direction of the coilconductor of the second line from the right of the coil portion C2 andface a part of the coil conductor in plan view.

Example 1B

FIG. 8 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 1B of an example 1B. Structures other thanthe wiring layer M0 are the same as those in the example 1. In the thinfilm balun 1B shown in FIG. 8, the electrode D2 of the capacitor D ispositioned so as to be parallel to an extending direction of a spacebetween the coil conductors of the first (outermost) and second linesfrom the right of the coil portion C2 and face a part of the coilconductors on both sides of the space in plan view.

Example 1C

FIG. 9 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 1C of an example 1C. Structures other thanthe wiring layer M0 are the same as those in the example 1. In the thinfilm balun 1C shown in FIG. 9, the electrode D2 of the capacitor D ispositioned so as to be parallel to an extending direction of the coilconductor of the third line from the right of the coil portion C2 andface a part of the coil conductor in plan view.

Example 1D

FIG. 10 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 1D of an example 1D. Structures other thanthe wiring layer M0 are the same as those in the example 1. In the thinfilm balun 1D shown in FIG. 10, the electrode D2 of the capacitor D inthe thin film balun 1B of the example 1B is shortened in length so thatthe electrode D2 of the capacitor D is shorter than one side of the coilportion C2 which is shaped in a rectangle.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 1 and 1A to 1D of the examples described abovewere measured by simulation. Evaluation target frequencies (resonantfrequencies fr) of a transmission signal were set at 2400 MHz to 2500MHz. FIG. 11 is a diagram showing passage characteristic evaluationresults, FIG. 12 is a diagram showing phase difference evaluationresults, and FIG. 13 is a diagram showing amplitude differenceevaluation results. In each of FIGS. 11 to 13, curves E1 and E1A to E1Drespectively indicate evaluation results of the thin film baluns 1 and1A to 1D.

The passage characteristics represent with how little loss a signalpasses through in an evaluation target frequency domain. 0 dB is idealpassage characteristics in the evaluation target frequency domain. Thephase difference is a difference in phase between two balanced signalsoutput from the balanced terminals BT1 and BT2, so that 180 deg is amore ideal phase balance. The amplitude difference is a difference inamplitude between two balanced signals output from the balancedterminals BT1 and BT2, so that 0 dB is a more ideal output balance.

These results demonstrate that the thin film balun of each of theexamples maintains excellent characteristics in the passagecharacteristics and the phase balance. The results also demonstrate thatthe amplitude balance can be adjusted by the position of the electrodeD2 of the capacitor D. In detail, it is demonstrated that the thin filmbaluns 1, 1A, 1B, and 1D in which the electrode D2 faces a part of acoil conductor closer to the outer peripheral coil conductor than theinner peripheral coil conductor in the coil portion C2 exhibit anexcellent amplitude balance as compared with the thin film balun 1C. Itis further demonstrated that the thin film baluns 1, 1B, and 1D in whichthe electrode D2 faces a part of the outer peripheral coil conductor inthe coil portion C2 exhibit an excellent amplitude balance as comparedwith the thin film balun 1A. In particular, the evaluation results ofthe thin film balun 1B demonstrate that the thin film balun in which theelectrode D2 of the capacitor D is positioned parallel to the extendingdirection of the space between the adjacent coil conductors in the coilportion C2 in plan view exhibits most excellent passage characteristicsand balance characteristics.

Example 2

In a thin film balun 2 of an example 2 in the first embodiment of thepresent invention, the coil portion C2 is made longer than the coilportion C1, and the electrode D2 of the capacitor D is positioned so asto face this extension of the coil portion C2. FIGS. 14 and 15 arehorizontal sectional views schematically showing the wiring layers M0and M1 of the thin film balun 2 of the example 2, respectively.

As shown in FIG. 14, the electrode D2 of the capacitor D is connected tothe ground terminal G and also extends from the ground terminal G towardthe unbalanced terminal UT, in the wiring layer M0.

As shown in FIG. 15, the coil portion C2 is formed longer than the coilportion C1 in the wiring layer M1, where an extension 12 c of the coilportion C2 extends in the same direction as a winding direction of thecoil portion C2 in the same layer as the coil portion C2, so as to besituated outside a region facing the balanced transmission line. Thisextension 12 c serves as the electrode D1 of the capacitor D. Theelectrode D1 which is a part of the coil portion C2 faces the electrodeD2 in the wiring layer M0. Other arrangements are the same as in theexample 1.

Example 2A

FIG. 16 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 2A of an example 2A. Structures other thanthe wiring layer M0 are the same as those in the example 2. In the thinfilm balun 2A shown in FIG. 16, the electrode D2 of the capacitor D ispositioned so as to be parallel to an extending direction of the coilconductor of the second line from the bottom of the coil portion C2 andface a part of the coil conductor in plan view.

Example 2B

FIG. 17 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 2B of an example 2B. Structures other thanthe wiring layer M0 are the same as those in the example 2. In the thinfilm balun 2B shown in FIG. 17, the electrode D2 of the capacitor D ispositioned so as to be parallel to an extending direction of a spacebetween the coil conductors of the second and third lines from thebottom of the coil portion C2 and face a part of the coil conductors onboth sides of the space in plan view.

Example 2C

FIG. 18 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 2C of an example 2C. Structures other thanthe wiring layer M0 are the same as those in the example 2. In the thinfilm balun 2C shown in FIG. 18, the electrode D2 of the capacitor D ispositioned so as to be parallel to an extending direction of a spacebetween the coil conductor of the first line (the extension 12 c) andthe coil conductor of the second line from the bottom of the coilportion C2 and face a part of the coil conductors on both sides of thespace in plan view.

Example 2D

FIG. 19 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 2D of an example 2D. Structures other thanthe wiring layer M0 are the same as those in the example 2. In the thinfilm balun 2D shown in FIG. 19, the electrode D2 of the capacitor D ispositioned so as to face a plurality of coil conductors 12 in a lowerright region of the coil portion C2. Here, the electrode D2 has the samearea as in the examples 2 and 2A to 2C.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 2 and 2A to 2D of the examples described abovewere measured by simulation. Evaluation target frequencies (resonantfrequencies fr) of a transmission signal were set at 2400 MHz to 2500MHz. FIG. 20 is a diagram showing passage characteristic evaluationresults, FIG. 21 is a diagram showing phase difference evaluationresults, and FIG. 22 is a diagram showing amplitude differenceevaluation results. In each of FIGS. 20 to 22, curves E2 and E2A to E2Drespectively indicate evaluation results of the thin film baluns 2 and2A to 2D.

These results demonstrate that the thin film balun of each of theexamples maintains excellent characteristics in the passagecharacteristics and the phase balance. The results also demonstrate thatthe amplitude balance can be adjusted by the position of the electrodeD2 of the capacitor D. In detail, the evaluation results of the thinfilm baluns 2 and 2A to 2D demonstrate that, by providing the electrodeD2 of the capacitor D to face the extension 12 c of the coil portion C2,the peak value of the resonant frequency fr shifts little even when theposition of the electrode D2 is adjusted. In particular, the evaluationresults of the thin film balun 2D demonstrate that the thin film balunin which the electrode D2 of the capacitor D is positioned facing theplurality of coil conductors 12 in the lower right region of the coilportion C2 in plan view exhibits a most excellent amplitude balance.

Differences in passage characteristics and balance characteristicsdepending on whether or not the coil portion C2 has the extension werealso evaluated. Thin film baluns of the following examples 1E and 1F andthe thin film baluns of the examples 2, 2A, and 2B were used for thisevaluation.

Examples 1E and 1F

FIG. 23 is a horizontal sectional view schematically showing the wiringlayer M1 of thin film baluns 1E and 1F of the examples 1E and 1F in thefirst embodiment of the present invention. As shown in FIG. 23, in thethin film baluns 1E and 1F, the coil portion C2 has no extension. Thoughnot shown, the thin film balun 1E has the same wiring layer M0 as thethin film balun 2A shown in FIG. 16, and the thin film balun 1F has thesame wiring layer M0 as the thin film balun 2B shown in FIG. 17.Structures other than the wiring layers M0 and M1 are the same as thosein the example 1.

Examples 2, 2A, and 2B

FIG. 24 is a horizontal sectional view confirmatorily showing the wiringlayer M1 of the thin film baluns 2, 2A, and 2B of the examples 2, 2A,and 2B in the first embodiment of the present invention described above.As shown in FIG. 24, the thin film baluns 2, 2A, and 2B have theextension 12 c serving as the electrode D1 of the capacitor D. Asmentioned earlier, the thin film balun 2 has the wiring layer M0 shownin FIG. 14, the thin film balun 2A has the wiring layer M0 shown in FIG.16, and the thin film balun 2B has the wiring layer M0 shown in FIG. 17.Structures other than the wiring layers M0 and M1 are the same as thosein the example 1.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 1E and 1F of the examples described above weremeasured by simulation. Evaluation target frequencies (resonantfrequencies fr) of a transmission signal were set at 2400 MHz to 2500MHz. FIG. 25 is a diagram showing passage characteristic evaluationresults, FIG. 26 is a diagram showing phase difference evaluationresults, and FIG. 27 is a diagram showing amplitude differenceevaluation results. The results of the thin film baluns 2, 2A, and 2Bmentioned earlier are also shown in each of FIGS. 25 to 27. In each ofFIGS. 25 to 27, curves E1E, E1F, E2, E2A, and E2B respectively indicateevaluation results of the thin film baluns 1E, 1F, 2, 2A, and 2B.

The evaluation results of the thin film baluns 1E and 1F demonstratethat, by providing the electrode D2 of the capacitor D to face theextension 12 c of the coil portion C2, the peak value of the resonantfrequency fr shifts little even when the position of the electrode D2 isadjusted.

Comparative Example

FIG. 28 is a horizontal sectional view schematically showing a wiringlayer B1 of a thin film balun of a comparative example. This wiringlayer B1 is provided between the insulating substrate 100 and the wiringlayer M0. As shown in FIG. 28, in the thin film balun of the comparativeexample, a capacitor K is positioned outside a range of a region of thecoil portion C2 in plan view. Though only an electrode K1 of thecapacitor K is shown in the drawing, an opposite electrode(corresponding to the electrode D2 of the examples in the firstembodiment) of the same shape as the electrode K1 is formed in thewiring layer M0 at a position facing the electrode K1. The electrode K1is connected to a through hole P near the ground terminal G by a wire51, and connected to the end 12 a of the coil portion C2 in the wiringlayer M1 via the through hole P. Meanwhile, the opposite electrodeformed in the wiring layer M0 is connected to the ground terminal G by awire 52.

(Characteristic Evaluation)

FIG. 29 shows results of evaluating passage characteristics (attenuationcharacteristics) of a transmission signal by simulation, for the thinfilm balun of the comparative example described above. In thesimulation, evaluation target frequencies (resonant frequencies fr) ofthe transmission signal were set at 2400 MHz to 2500 MHz. The passagecharacteristics of the thin film balun were evaluated with desiredspecifications being less than 1 dB in attenuation in this frequencyrange.

In FIG. 29, a curve R indicates the evaluation results of the thin filmbalun of the comparative example. The results shown in FIG. 29demonstrate that the thin film balun of the comparative example has alarger attenuation in the frequency domain of the desired specificationsin passage characteristics than the thin film balun according to thepresent invention, and fails to satisfy the desired specifications.

As noted earlier, the present invention is not limited to the aboveexamples, and various changes can be made without departing from thescope of the present invention. For example, the arrangement of theunbalanced terminal UT, the balanced terminals BT1 and BT2, and theground terminal G is not limited to the positions shown in the drawings.Moreover, the number of layers of the multilayer wiring structureforming the thin film balun may be smaller or larger than the numbershown in the drawings. Besides, the wiring layers on the insulatingsubstrate 100 may be reversed in order. Furthermore, various coilarrangements may be employed without departing from the scope of thepresent invention.

According to the thin film balun in the first embodiment of the presentinvention, a part of the line portion constituting the unbalancedcircuit serves as one electrode of the capacitor connected to theunbalanced circuit, so that the thin film balun can be made smaller andthinner while maintaining required balun characteristics. Such a thinfilm balun is widely applicable, in particular to wireless communicationdevices, apparatuses, modules, and systems which are required to besmaller and thinner, facilities including them, and manufacturingthereof.

Second Embodiment

A thin film balun in a second embodiment corresponding to the secondaspect of the present invention is described below. FIG. 30 is anequivalent circuit diagram showing a structure of the thin film balun inthe second embodiment of the present invention. A thin film balun 3includes the unbalanced transmission line (unbalanced circuit) UL inwhich the line portion L1 (first line portion) and the line portion L2(second line portion) are connected in series, and the balancedtransmission line (balanced circuit) BL in which the line portion L3(third line portion) and the line portion L4 (fourth line portion) areconnected in series. The line portions L1 and L3 form magnetic coupling,and the line portions L2 and L4 form magnetic coupling.

In the thin film balun 3, the end of the line portion L1 other than theend connected to the line portion L2 is connected to the unbalancedterminal UT, and the end of the line portion L2 other than the endconnected to the line portion L1 is connected to the ground terminal G(ground potential) via the capacitor D which is a C component(capacitance component). The capacitor D is formed by the electrode D1connected to the other end of the line portion L2 and the electrode D2connected to the ground terminal G where the electrodes D1 and D2 faceeach other via an appropriate dielectric. On the other hand, the end ofthe line portion L3 other than the end connected to the line portion L4is connected to the balanced terminal BT1, and the end of the lineportion L4 other than the end connected to the line portion L3 isconnected to the balanced terminal BT2. Moreover, the connected ends ofthe line portions L3 and L4 are connected to the ground terminal G.

Lengths of the above-mentioned line portions L1 to L4 vary depending onspecifications of the thin film balun 3. For example, the lengths may beset so as to form a quarter-wavelength (λ/4) resonator circuit of atransmission signal which is subject to conversion. Moreover, the lineportions L1 to L4 may be arbitrarily shaped so long as theabove-mentioned magnetic coupling is formed. Example shapes include aspiral (coil form), a zigzag, a straight line, and a curved line.

A basic operation of the thin film balun 3 is described below, withreference to FIG. 30. In the thin film balun 3, when an unbalancedsignal is input to the unbalanced terminal UT, the unbalanced signalpropagates through the line portions L1 and L2. By the magnetic coupling(first magnetic coupling) of the line portions L1 and L3 and themagnetic coupling (second magnetic coupling) of the line portions L2 andL4, the input unbalanced signal is converted to two balanced signalsthat differ in phase by 180° (π), and the two balanced signals areoutput from the balanced terminals BT1 and BT2. A converting operationfrom balanced signals to an unbalanced signal is the reverse of theabove-mentioned converting operation from an unbalanced signal tobalanced signals.

A wiring structure of the thin film balun in the second embodiment isdescribed below. FIG. 31 is a vertical sectional view schematicallyshowing the wiring structure of the thin film balun 3. As shown in FIG.31, the wiring layers M1, M2, and M3 are formed in this order on theinsulating substrate 100 of alumina or the like. For instance, theunbalanced transmission line UL mentioned above is formed by the wiringlayer M1, and the balanced transmission line BL is formed by the wiringlayer M2. Dielectric layers are formed between wires in the same wiringlayer and between different wiring layers. For example, the dielectriclayer 102 made of silicon nitride is formed between the wiring layers M1and M2. Polyimide or the like is used as the dielectric layer 101 inother parts. Note that the materials are not limited to the above, andnot only inorganic insulators such as silicon nitride, alumina, andsilica but also organic insulators such as polyimide and epoxy resin maybe selected according to need. The unbalanced terminal UT, the balancedterminals BT1 and BT2, and the ground terminal G are formed through alldielectric layers. Thus, the thin film balun 3 is composed of a thinfilm multilayer structure formed on the insulating substrate 100.

As shown in FIG. 31, in the second embodiment, the above-mentionedcapacitor (C component) D is provided between the wiring layer M1 (firstlayer) that includes the unbalanced transmission line UL and the wiringlayer M2 (second layer) that faces the wiring layer M1 via thedielectric layer 102 and includes the balanced transmission line BL.

A pattern of each of the wiring layers M1, M2, and M3 of the thin filmbalun in the second embodiment is described in detail below. Coilportions are used as the line portions L1 to L4 in each of the followingexamples.

Example 1

FIGS. 32 to 34 are each a horizontal sectional view schematicallyshowing a different one of the wiring layers of the thin film balun 3 ofan example 1 in the second embodiment of the present invention. As shownin FIGS. 32 to 34, the unbalanced terminal UT, the balanced terminalsBT1 and BT2, and the ground terminal G are formed in all of the wiringlayers M1 to M3. Each of the terminals UT, BT1, BT2, and G iselectrically connected between different layers via a through hole P.Note that all through holes P shown in FIGS. 32 to 34 are plated with ametal conductor for electrical conduction of upper and lower layers. Astructure of each wiring layer is described in detail below.

As shown in FIG. 32, the coil portion C1 (first coil portion, first lineportion) and the coil portion C2 (second coil portion, second lineportion) that constitute the unbalanced transmission line UL are formedadjacent to each other in the wiring layer M1 on the insulatingsubstrate 100. Each of the coil portions C1 and C2 forms an equivalentof a quarter-wavelength (λ/4) resonator. The outer end 11 a of the coilconductor 11 constituting the coil portion C1 is connected to theunbalanced terminal UT, and the inner end 11 b of the coil conductor 11is connected to a through hole P. The inner end 12 b of the coilconductor 12 constituting the coil portion C2 is connected to a throughhole P. The electrode D1 of the capacitor D extends from the outer end(outer peripheral end) 12 a of the coil conductor 12 (the end referredto here does not denote an end of a physical shape, but denotes an endof a coil equivalent to the coil portion C1), in a state of being woundin a direction opposite to a winding direction of the coil conductor 12.Since the electrode D1 of the capacitor D is placed in the same layer asthe coil portion C2, the electrode D1 is inevitably situated outside therange of the region of the coil portion C2.

As shown in FIG. 33, the coil portion C3 (third coil portion, third lineportion) and the coil portion C4 (fourth coil portion, fourth lineportion) that constitute the balanced transmission line BL are formedadjacent to each other in the wiring layer M2. Each of the coil portionsC3 and C4 forms an equivalent of a quarter-wavelength (λ/4) resonator,as with the coil portions C1 and C2. The coil portions C3 and C4 of thebalanced transmission line BL are positioned facing the coil portions C1and C2 of the unbalanced transmission line UL respectively, and thefacing portions are magnetically coupled to form couplers. The outer end21 a of the coil conductor 21 constituting the coil portion C3 isconnected to the balanced terminal BT1, and the inner end 21 b of thecoil conductor 21 is connected to a through hole P. The outer end 22 aof the coil conductor 22 constituting the coil portion C4 is connectedto the balanced terminal BT2, and the inner end 22 b of the coilconductor 22 is connected to a through hole P. In addition, theelectrode D2 facing the electrode D1 is formed in the wiring layer M2,and an end of the electrode D2 is connected to the ground terminal G.

As shown in FIG. 34, the wire 31 for connecting the coil portions C3 andC4 to the ground terminal G and the wire 32 for connecting the coilportions C1 and C2 are formed in the wiring layer M3. The wire 31 is abranch wire formed so as to connect two through holes P to the groundterminal G. The wire 31 is connected to the end 21 b of the coilconductor 21 and the end 22 b of the coil conductor 22 formed in thewiring layer M2, via the two through holes P. The wire 32 is connectedto the end 11 b of the coil conductor 11 and the end 12 b of the coilconductor 12 formed in the wiring layer M1, via through holes P.

Thus, in the example 1, the thin film balun 3 forming the equivalentcircuit shown in FIG. 30 is obtained by a multilayer wiring structure inwhich the two coil portions C1 and C2 constituting the unbalancedtransmission line UL and the electrode D1 of the capacitor D are formedin the wiring layer M1 which is one layer, the two coil portions C3 andC4 constituting the balanced transmission line BL and the electrode D2of the capacitor D are formed in the wiring layer M2 which is anotherlayer adjacent to the wiring layer M1, and the wire 32 connecting thecoil portions C1 and C2 and the wire 31 connecting the coil portions C3and C4 are formed in the wiring layer M3 which is another layer adjacentto the wiring layer M2 on an opposite side to the wiring layer M1.

Comparative Example 1

In a thin film balun 3A of a comparative example 1, the capacitor D isprovided between wiring layers other than the wiring layer M1 in whichthe unbalanced transmission line UL is formed and the wiring layer M2 inwhich the balanced transmission line BL is formed. FIGS. 35 to 38 arehorizontal sectional views schematically showing the wiring layers B1,M0, M1, and M2 of the thin film balun 3A of the comparative example 1.Here, the wiring layers B1, M0, M1, and M2 are formed in this order onthe insulating substrate 100.

As shown in FIG. 35, the electrode D2 of the capacitor D is formed inthe wiring layer B1 on the insulating substrate 100, in a region outsidethe coil portion C1 and near the balanced terminal BT1 in plan view. Theelectrode D2 is connected to the ground terminal G via a wire 41.

As shown in FIG. 36, the electrode D1 facing the electrode D2 is formedin the wiring layer M0. The electrode D1 is connected to the end 12 a ofthe coil portion C2 in the wiring layer M1, via the wire 51 and athrough hole P.

As shown in FIG. 37, the coil portions C1 and C2 that constitute theunbalanced transmission line UL are formed adjacent to each other in thewiring layer M1. The wiring layer M1 in the comparative example 1 is thesame as that in the second embodiment, except that the electrode D1 ofthe capacitor D is not formed. The outer end 12 a of the coil conductor12 constituting the coil portion C2 is connected to the electrode D1 ofthe capacitor D provided in the wiring layer M0, via a through hole P.

As shown in FIG. 38, the coil portions C3 and C4 that constitute thebalanced transmission line BL are formed adjacent to each other in thewiring layer M2. The wiring layer M2 in the comparative example 1 is thesame as that in the second embodiment, except that the electrode D2 ofthe capacitor D is not formed.

Though not shown, the wiring layer M3 is provided on the wiring layer M2and the wires 31 and 32 are formed in the wiring layer M3, as in thesecond embodiment.

Thus, in the comparative example 1, the thin film balun 3A forming theequivalent circuit shown in FIG. 30 is obtained by a multilayer wiringstructure in which the two coil portions C1 and C2 constituting theunbalanced transmission line UL are formed in the wiring layer M1 whichis one layer, the two coil portions C3 and C4 constituting the balancedtransmission line BL are formed in the wiring layer M2 which is anotherlayer adjacent to the wiring layer M1, the wire 32 connecting the coilportions C1 and C2 and the wire 31 connecting the coil portions C3 andC4 are formed in the wiring layer M3 which is another layer adjacent tothe wiring layer M2 on an opposite side to the wiring layer M1, theelectrode D1 of the capacitor D is formed in the wiring layer M0 whichis another layer adjacent to the wiring layer M1 on an opposite side tothe wiring layer M2, and the electrode D2 of the capacitor D is formedin the wiring layer B1 which is another layer adjacent to the wiringlayer M0 on an opposite side to the wiring layer M1.

Comparative Example 2

FIGS. 39 and 40 are horizontal sectional views schematically showing thewiring layers B1 and M0 of a thin film balun 3B of a comparative example2, respectively. The wiring layers M1 to M3 other than the wiring layersB1 and M0 have the same structures as in the comparative example 1.

As shown in FIG. 39, the electrode D2 of the capacitor D is formed inthe wiring layer B1 of the thin film balun 3B, in a region outside thecoil portion C1 and near the unbalanced terminal UT in plan view. Theelectrode D2 is connected to the ground terminal G via the wire 41.

As shown in FIG. 40, the electrode D1 facing the electrode D2 is formedin the wiring layer M0. The electrode D1 is connected to the end 12 a ofthe coil portion C2 in the wiring layer M1, via the wire 51 and athrough hole P.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 3, 3A, and 3B described above were measured bysimulation. Evaluation target frequencies (resonant frequencies fr) of atransmission signal were set at 2400 MHz to 2500 MHz. FIG. 41 is adiagram showing passage characteristic evaluation results, FIG. 42 is adiagram showing phase difference evaluation results, and FIG. 43 is adiagram showing amplitude difference evaluation results. In each ofFIGS. 41 to 43, curves E3, E3A, and E3B respectively indicate evaluationresults of the thin film baluns 3, 3A, and 3B.

The passage characteristics represent with how little loss a signalpasses through in an evaluation target frequency domain. 0 dB is idealpassage characteristics in the evaluation target frequency domain. Thephase difference is a difference in phase between two balanced signalsoutput from the balanced terminals BT1 and BT2, so that 180 deg is amore ideal phase balance. The amplitude difference is a difference inamplitude between two balanced signals output from the balancedterminals BT1 and BT2, so that 0 dB is a more ideal output balance.

These results demonstrate that the thin film balun in the secondembodiment maintains excellent characteristics in both the passagecharacteristics and the balance characteristics. In particular, theresults demonstrate that the thin film balun in the second embodimentexhibits an excellent amplitude balance as compared with the thin filmbaluns 3A and 3B of the comparative examples.

Comparative Example 3

FIG. 44 is a horizontal sectional view schematically showing the wiringlayer B1 of a thin film balun of a comparative example 1 This wiringlayer B1 is provided between the insulating substrate 100 and the wiringlayer M1. As shown in FIG. 44, in the thin film balun of the comparativeexample, the electrode K1 of the capacitor K is formed in the wiringlayer B1 other than the wiring layer M1 in which the unbalancedtransmission line UL is formed. The electrode K1 is positioned outsidethe range of the region of the coil portion C1. Besides, unlike thesecond embodiment, the wiring layer M0 (not shown) is provided betweenthe wiring layer B1 and the wiring layer M1, and an opposite electrodeK2 (corresponding to the electrode D2 in the second embodiment) of thesame shape as the electrode K1 is formed in the wiring layer M0 at aposition facing the electrode K1. The electrode K1 is connected to athrough hole P near the ground terminal G by the wire 51, and connectedto the end 12 a of the coil portion C2 in the wiring layer M1 via thethrough hole P. The opposite electrode formed in the wiring layer M0 isconnected to the ground terminal G by the wire 52.

(Characteristic Evaluation)

FIG. 45 shows results of evaluating passage characteristics (attenuationcharacteristics) of a transmission signal by simulation, for the thinfilm balun of the comparative example 3. In the simulation, evaluationtarget frequencies (resonant frequencies fr) of the transmission signalwere set at 2400 MHz to 2500 MHz. The passage characteristics of thethin film balun were evaluated with desired specifications being lessthan 1 dB in attenuation in this frequency range.

In FIG. 45, a curve R indicates the evaluation results of the thin filmbalun of the comparative example 3. The results shown in FIG. 45demonstrate that the thin film balun of the comparative example 3 has alarger attenuation in the frequency domain of the desired specificationsin passage characteristics than the thin film balun in the secondembodiment, and fails to satisfy the desired specifications.

Example 3C

FIG. 46 is a vertical sectional view schematically showing a wiringstructure of a thin film balun 3C of an example 3C. As shown in FIG. 46,the wiring layers M1, M2, and M3 are formed in this order on theinsulating substrate 100 of alumina or the like. The wiring layers M1,M2, and M3 have the same structures as in the second embodiment.Dielectric layers 102 and 102A made of, for example, silicon nitride areformed between the wiring layers M1 and M2. As shown in FIG. 46, in thethin film balun 3C of the example 3C, the dielectric layer 102A in apart where the capacitance coupling of the capacitor D is formed has asmaller film thickness than the dielectric layer 102 in a part where themagnetic coupling between the unbalanced transmission line UL and thebalanced transmission line BL is formed.

According to this structure, it is possible to form the capacitor Dhaving the dielectric layer 102A of a film thickness different from afilm thickness required for the magnetic coupling between the unbalancedtransmission line UL and the balanced transmission line BL. This allowsa desired capacitance C to be introduced in the resonant circuit of thethin film balun, thereby suppressing an increase in resonant frequency.

As noted earlier, the present invention is not limited to the aboveexamples, and various changes can be made without departing from thescope of the present invention. For example, instead of extending theouter peripheral end of the coil portion C2, a part of the outerperiphery of the coil portion C2 may be used as the electrode D1 of thecapacitor. Moreover, the arrangement of the unbalanced terminal UT, thebalanced terminals BT1 and BT2, and the ground terminal G is not limitedto the positions shown in the drawings. Besides, the wiring layers onthe insulating substrate 100 may be reversed in order. Furthermore,various coil arrangements may be employed without departing from thescope of the present invention.

According to the thin film balun in the second embodiment of the presentinvention, the C component connected to the second line portion isprovided between the first layer including the unbalanced transmissionline and the second layer including the balanced transmission line, sothat the thin film balun can be made smaller and thinner whilemaintaining required balun characteristics. Such a thin film balun iswidely applicable, in particular to wireless communication devices,apparatuses, modules, and systems which are required to be smaller andthinner, facilities including them, and manufacturing thereof.

Third Embodiment

A thin film balun in a third embodiment corresponding to the thirdaspect of the present invention is described below. FIG. 47 is anequivalent circuit diagram showing a structure of the thin film balun inthe third embodiment of the present invention. A thin film balun 4includes the unbalanced transmission line (unbalanced circuit) UL inwhich the line portion L1 (first line portion) and the line portion L2(second line portion) are connected in series, and the balancedtransmission line (balanced circuit) BL in which the line portion L3(third line portion) and the line portion L4 (fourth line portion) areconnected in series. The line portions L1 and L3 form electromagneticcoupling, and the line portions L2 and L4 form electromagnetic coupling.

In the thin film balun 4, the end of the line portion L1 other than theend connected to the line portion L2 is connected to the unbalancedterminal UT. The end of the line portion L2 other than the end connectedto the line portion L1 is an open end, and a part of the line portion L2except the open end is connected to the unbalanced terminal UT via acapacitor D1. On the other hand, the end of the line portion L3 otherthan the end connected to the line portion L4 is connected to thebalanced terminal BT1, and the end of the line portion L4 other than theend connected to the line portion L3 is connected to the balancedterminal BT2. Moreover, the connected ends of the line portions L3 andL4 are connected to the ground terminal G.

Lengths of the above-mentioned line portions L1 to L4 vary depending onspecifications of the thin film balun 4. For example, the lengths may beset so as to form a quarter-wavelength (λ/4) resonator circuit of atransmission signal which is subject to conversion. Moreover, the lineportions L1 to L4 may be arbitrarily shaped so long as theabove-mentioned electromagnetic coupling is formed. Example shapesinclude a spiral (coil form), a zigzag, a straight line, and a curvedline.

A basic operation of the thin film balun 4 is described below, withreference to FIG. 47. When an unbalanced signal is input to theunbalanced terminal UT, the unbalanced signal propagates through theline portions L1 and L2. By the electromagnetic coupling (firstelectromagnetic coupling) of the line portions L1 and L3 and theelectromagnetic coupling (second electromagnetic coupling) of the lineportions L2 and L4, the input unbalanced signal is converted to twobalanced signals that have the same frequency as the unbalanced signaland differ in phase by 180° (π), and the two balanced signals are outputfrom the balanced terminals BT1 and BT2. On the other hand, when twobalanced signals of the same magnitude that have a predeterminedfrequency and differ in phase by 180° are input to the balancedterminals BT1 and BT2, an unbalanced signal of the same frequency as thebalanced signals is output from the unbalanced terminal UT.

A wiring structure of the thin film balun in the third embodiment isdescribed below. FIG. 48 is a vertical sectional view schematicallyshowing the wiring structure of the thin film balun 4. As shown in FIG.48, the wiring layers M0, M1, M2, and M3 are formed in this order on theinsulating substrate 100 of alumina or the like. For instance, acapacitor electrode DE1 is formed by the wiring layer M0, the unbalancedtransmission line UL is formed by the wiring layer M1, the balancedtransmission line BL is formed by the wiring layer M2, and theconnection wires for connecting the line portions in the balancedtransmission line BL and the unbalanced transmission line UL are formedby the wiring layer M3. Dielectric layers 101 to 105 are formed betweenwires in the same wiring layer and between different wiring layers. Forexample, silicon nitride is used as the dielectric layer 102 between thewiring layers M0 and M1 where the capacitor D is formed, and thedielectric layer 104 between the wiring layers M1 and M2 where theelectromagnetic coupling between the unbalanced transmission line UL andthe balanced transmission line BL is formed. For example, alumina isused as the dielectric layers 101 and 103 covering the wiring layers M0and M1. For example, polyimide is used as the dielectric layer 105covering the wiring layers M2 and M3. Note that the materials of theselayers are not limited to the above, and not only inorganic insulatorssuch as silicon nitride, alumina, and silica but also organic insulatorssuch as polyimide and epoxy resin may be selected according to need. Theunbalanced terminal UT, the balanced terminals BT1 and BT2, and theground terminal G are formed through all dielectric layers. Thus, thethin film balun 4 is composed of a thin film multilayer structure formedon the insulating substrate 100.

As shown in FIG. 48, in the third embodiment, the above-mentionedcapacitor D is formed between the wiring layer M1 including theunbalanced transmission line UL and the wiring layer M0 facing thewiring layer M1 via the dielectric layer 102 and including the capacitorelectrode DE1.

A pattern of each of the wiring layers M0, M1, M2, and M3 of the thinfilm balun in the third embodiment is described in detail below. Coilportions are used as the line portions L1 to L4 in each of the followingexamples.

Example 1A

FIGS. 49 to 52 are each a horizontal sectional view schematicallyshowing a different one of the wiring layers of a thin film balun 4A ofan example 1A. As shown in FIGS. 49 to 52, the unbalanced terminal UT,the balanced terminals BT1 and BT2, and the ground terminal G are formedin all of the wiring layers M0 to M3. Each of the terminals UT, BT1,BT2, and G is electrically connected between different layers via athrough hole P. Note that all through holes P shown in FIGS. 49 to 52are plated with a metal conductor for electrical conduction of upper andlower layers. A structure of each wiring layer is described in detailbelow.

As shown in FIG. 49, the capacitor electrode DE1 of the capacitor D1 isformed in the wiring layer M0 on the insulating substrate 100, at aposition facing a part of the coil portion C2 in the wiring layer M1.The capacitor electrode DE1 is connected to the unbalanced terminal UT.The capacitor electrode DE1 of the capacitor D1 is positioned so as toface the coil conductor 12 of the first line from the left of the coilportion C2 in plan view.

As shown in FIG. 50, the coil portion C1 (first line portion) and thecoil portion C2 (second line portion) that constitute the unbalancedtransmission line UL are formed adjacent to each other in the wiringlayer M1. Each of the coil portions C1 and C2 forms an equivalent of aquarter-wavelength (λ/4) resonator. The outer end 11 a of the coilconductor 11 constituting the coil portion C1 is connected to theunbalanced terminal UT, and the inner end 11 b of the coil conductor 11is connected to a through hole P. The inner end 12 b of the coilconductor 12 constituting the coil portion C2 is connected to a throughhole P, and the outer end 12 a (open end) of the coil conductor 12 isopen. A part of the coil conductor 12 except the outer end 12 a servesas a capacitor electrode of the capacitor D1, and faces the capacitorelectrode DE1 in the wiring layer M0. Moreover, the coil conductor 12constituting the coil portion C2 has a larger width than the coilconductor 11 constituting the coil portion C1, as shown in FIG. 50. As aresult of conducting intense study, the present inventor has found that,in the case of inserting the capacitor D1 so that a part of the coilportion C2 serves as an electrode of the capacitor, excellent passagecharacteristics and balance characteristics can be obtained by formingthe coil conductor 12 wider than the coil conductor 11. Note, however,that there is no limitation on the widths and the numbers of turns ofthe coil conductors 12 and 11, and the widths and the numbers of turnsof the coil conductors 12 and 11 may be equal or different.

As shown in FIG. 51, the coil portion C3 (third line portion) and thecoil portion C4 (fourth line portion) that constitute the balancedtransmission line BL are formed adjacent to each other in the wiringlayer M2. Each of the coil portions C3 and C4 forms an equivalent of aquarter-wavelength (λ/4) resonator, as with the coil portions C1 and C2.The coil portions C3 and C4 of the balanced transmission line BL arepositioned facing the coil portions C1 and C2 of the unbalancedtransmission line UL respectively, and the facing portions areelectromagnetically coupled to form couplers. The outer end 21 a of thecoil conductor 21 constituting the coil portion C3 is connected to thebalanced terminal BT1, and the inner end 21 b of the coil conductor 21is connected to a through hole P. The outer end 22 a of the coilconductor 22 constituting the coil portion C4 is connected to thebalanced terminal BT2, and the inner end 22 b of the coil conductor 22is connected to a through hole P. Moreover, the coil conductor 22 has alarger width than the coil conductor 21 and a larger number of turnsthan the coil conductor 21, as shown in FIG. 51. As mentioned above, asa result of conducting intense study, the present inventor has foundthat, in the case of inserting the capacitor D1 so that a part of thecoil portion C2 serves as an electrode of the capacitor, excellentpassage characteristics and balance characteristics can be obtained bysetting the widths and the numbers of turns of the coil conductors 21and 22 in the above way. Note, however, that there is no limitation onthe widths and the numbers of turns of the coil conductors 22 and 21,and the widths and the numbers of turns of the coil conductors 22 and 21may be equal or different.

As shown in FIG. 52, the wire 31 for connecting the coil portions C3 andC4 to the ground terminal G and the wire 32 for connecting the coilportions C1 and C2 are formed in the wiring layer M3. The wire 31 is abranch wire formed so as to connect two through holes P to the groundterminal G. The wire 31 is connected to the end 21 b of the coilconductor 21 and the end 22 b of the coil conductor 22 formed in thewiring layer M2, via the two through holes P. The wire 32 is connectedto the end 11 b of the coil conductor 11 and the end 12 b of the coilconductor 12 formed in the wiring layer M1, via through holes P.

Thus, in the example 1A, the thin film balun 4A forming the equivalentcircuit shown in FIG. 47 is obtained by a multilayer wiring structure inwhich the two coil portions C1 and C2 constituting the unbalancedtransmission line are formed in the wiring layer M1 which is one layer,the two coil portions C3 and C4 constituting the balanced transmissionline are formed in the wiring layer M2 which is another layer adjacentto the wiring layer M1, the wire 32 connecting the coil portions C1 andC2 and the wire 31 connecting the coil portions C3 and C4 are formed inthe wiring layer M3 which is another layer adjacent to the wiring layerM2 on an opposite side to the wiring layer M1, and the capacitorelectrode DE1 facing the part of the coil conductor 12 in the coilportion C2 to form the capacitor D1 is formed in the wiring layer M0which is another layer adjacent to the wiring layer M1 on an oppositeside to the wiring layer M2.

Example 1B

FIG. 53 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 4B of an example 1B. Structures other thanthe wiring layer M0 are the same as those in the example 1A. In the thinfilm balun 4B shown in FIG. 53, the length of the capacitor electrodeDE1 (the length of the part that substantially serves as a capacitorelectrode) is half the length in the example 1A. Thus, in the thin filmbalun of the example 1B, the capacitance introduced in the resonantcircuit of the thin film balun is set to half the capacitance in theexample 1A.

(Characteristic Evaluation)

Passage characteristics (insertion loss) of each of the thin film baluns4A and 4B described above were measured by simulation. Evaluation targetfrequencies (resonant frequencies fr) of a transmission signal were setat 2400 MHz to 2500 MHz. FIG. 54 is a diagram showing passagecharacteristic evaluation results. In FIG. 54, curves E4A and E4Brespectively indicate evaluation results of the thin film baluns 4A and4B. Meanwhile, a curve RO in FIG. 54 indicates evaluation results of athin film balun of a comparative example that does not include thecapacitor electrode DE1. The passage characteristics represent with howlittle loss a signal passes through in an evaluation target frequencydomain. 0 dB is ideal passage characteristics in the evaluation targetfrequency domain.

The results shown in FIG. 54 demonstrate that the thin film baluns 4Aand 4B of the examples that include the capacitor D1 have the resonantfrequency shifted toward lower frequencies, as compared with the balunof the comparative example that does not include the capacitor. Theresults further demonstrate that the amount of shift is influenced bythe capacitance of the introduced capacitor D1.

Example 1C

FIG. 55 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 4C of an example 1C. Structures other thanthe wiring layer M0 are the same as those in the example 1A. In the thinfilm balun 4C shown in FIG. 55, the capacitor electrode DE1 ispositioned extending vertically so as to face the coil conductor 12 ofthe second line from the left of the coil portion C2 in plan view. Thearea of the capacitor electrode DE1 in the part facing the coil portionC2 (the area that substantially serves as a capacitor electrode) is setto be the same as in the example 1A.

Example 1D

FIG. 56 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 4D of an example 1D. Structures other thanthe wiring layer M0 are the same as those in the example 1A. In the thinfilm balun 4D shown in FIG. 56, the capacitor electrode DE1 ispositioned extending horizontally so as to face the coil conductor 12 ofthe first line from the bottom of the coil portion C2 in plan view. Thearea of the capacitor electrode DE1 in the part facing the coil portionC2 (the area that substantially serves as a capacitor electrode) is setto be the same as in the example 1A.

Example 1E

FIG. 57 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 4E of an example 1E. Structures other thanthe wiring layer M0 are the same as those in the example 1A. In the thinfilm balun 4E shown in FIG. 57, the capacitor electrode DE1 ispositioned extending vertically so as to face the coil conductor 12 ofthe second line from the right of the coil portion C2 in plan view. Thearea of the capacitor electrode DE1 in the part facing the coil portionC2 (the area that substantially serves as a capacitor electrode) is setto be the same as in the example 1A.

Example 1F

FIG. 58 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 4F of an example 1F. Structures other thanthe wiring layer M0 are the same as those in the example 1A. In the thinfilm balun 4F shown in FIG. 58, the capacitor electrode DE1 ispositioned extending vertically so as to face the open end of the coilconductor 12 constituting the coil portion C2 in plan view. The area ofthe capacitor electrode DE1 in the part facing the coil portion C2 (thearea that substantially serves as a capacitor electrode) is set to bethe same as in the example 1A.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 4C to 4F described above were measured bysimulation. Evaluation target frequencies (resonant frequencies fr) of atransmission signal were set at 2400 MHz to 2500 MHz. FIG. 59 is adiagram showing passage characteristic evaluation results, FIG. 60 is adiagram showing phase difference evaluation results, and FIG. 61 is adiagram showing amplitude difference evaluation results. In each ofFIGS. 59 to 61, curves E4A and E4C to E4F respectively indicateevaluation results of the thin film baluns 4A and 4C to 4F.

The phase difference is a difference in phase between two balancedsignals output from the balanced terminals BT1 and BT2, so that 180 degis a more ideal phase balance. The amplitude difference is a differencein amplitude between two balanced signals output from the balancedterminals BT1 and BT2, so that 0 dB is a more ideal output balance.

These results demonstrate that the thin film baluns 4A and 4C to 4F ofthe examples maintain excellent characteristics in both the passagecharacteristics and the balance characteristics. The results alsodemonstrate that, of the examples, the thin film baluns 4A and 4C to 4Ein which the capacitor electrode DE1 faces a part of the coil conductor12 other than the open end exhibit excellent balance characteristics ascompared with the thin film balun 4F in which the capacitor electrodeDE1 faces the open end of the coil portion C2.

Example 2

FIG. 62 is an equivalent circuit diagram showing a structure of a thinfilm balun of an example 2 in the third embodiment of the presentinvention. In a thin film balun 5 of the example 2, not only thecapacitor D1 is introduced between the unbalanced terminal UT and theline portion L2, but also a capacitor D2 is introduced between theunbalanced terminal UT and the line portion L1.

Example 2A

A pattern of the wiring layer M0 in the example 2 of the thin film balunhaving the circuit structure shown in FIG. 62 is described in detailbelow. FIG. 63 is a horizontal sectional view schematically showing thewiring layer M0 of a thin film balun 5A of an example 2A according tothe present invention. Structures other than the wiring layer M0 are thesame as those in the example 1A. In the thin film balun 5A shown in FIG.63, two capacitor electrodes DE1 and DE2 connected to the unbalancedterminal UT are formed in the wiring layer M0. The capacitor electrodeDE1 is positioned in the same way as in the example 1E. The capacitorelectrode DE2 is positioned extending vertically so as to face the coilconductor 11 of the second line from the left of the coil portion C1 inplan view.

Example 2B

FIG. 64 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 5B of an example 2B. Structures other thanthe wiring layer M0 are the same as those in the example 1A. In the thinfilm balun 5B shown in FIG. 64, the two capacitor electrodes DE1 and DE2connected to the unbalanced terminal UT are formed in the wiring layerM0. The capacitor electrode DE1 is positioned in the same way as in theexample 1E. The capacitor electrode DE2 is branched to have a largerelectrode area than in the example 2A.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 5A and 5B described above were measured bysimulation. Evaluation target frequencies (resonant frequencies fr) of atransmission signal were set at 2400 MHz to 2500 MHz. FIG. 65 is adiagram showing passage characteristic evaluation results, FIG. 66 is adiagram showing phase difference evaluation results, and FIG. 67 is adiagram showing amplitude difference evaluation results. In each ofFIGS. 65 to 67, curves E5A and E5B respectively indicate evaluationresults of the thin film baluns 5A and 5B. Meanwhile, a curve E4E ineach of FIGS. 65 to 67 indicates evaluation results of the thin filmbalun 4E of the example that does not include the capacitor electrodeDE2.

The results shown in FIGS. 66 and 67 demonstrate that the thin filmbaluns 5A and 5B of the examples exhibit excellent balancecharacteristics as compared with the thin film balun 4E. Moreover, theresults shown in FIG. 65 demonstrate that the thin film balun 5B has theresonant frequency shifted slightly toward lower frequencies as comparedwith the thin film balun 4E.

As noted earlier, the present invention is not limited to the aboveexamples, and various changes can be made without departing from thescope of the present invention. For example, though the part of the coilportion C1 or C2 constituting the unbalanced transmission line UL servesas a capacitor electrode in the examples, a layer of another capacitorelectrode connected to the coil portion C1 or C2 may be formed betweenthe coil portion C1 or C2 and the capacitor electrode DE2 or DE1.Moreover, the arrangement of the unbalanced terminal UT, the balancedterminals BT1 and BT2, and the ground terminal G is not limited to thepositions shown in the drawings. Besides, the wiring layers on theinsulating substrate 100 may be reversed in order. Furthermore, variouscoil arrangements may be employed without departing from the scope ofthe present invention.

According to the thin film balun in the third embodiment of the presentinvention, the capacitor is introduced between the unbalanced terminaland a part of the unbalanced transmission line facing the balancedtransmission line, so that the thin film balun can be made smaller andthinner while maintaining required balun characteristics. Such a thinfilm balun is widely applicable, in particular to wireless communicationdevices, apparatuses, modules, and systems which are required to besmaller and thinner, facilities including them, and manufacturingthereof.

Fourth Embodiment

A thin film balun in a fourth embodiment corresponding to the fourthaspect of the present invention is described below. FIG. 68 is anequivalent circuit diagram showing a structure of a preferred embodimentof the thin film balun in the fourth embodiment of the presentinvention. A thin film balun 6 includes the unbalanced transmission line(unbalanced circuit) UL in which the line portion L1 (first lineportion) and the line portion L2 (second line portion) are connected inseries, and the balanced transmission line (balanced circuit) BL inwhich the line portion L3 (third line portion) and the line portion L4(fourth line portion) are connected in series. The line portions L1 andL3 form magnetic coupling, and the line portions L2 and L4 form magneticcoupling.

In the thin film balun 6, the end of the line portion L1 other than theend connected to the line portion L2 is connected to the unbalancedterminal UT, and a part of the line portion L1 is connected to theground terminal G (ground potential) via the capacitor D which is a Ccomponent (capacitance component). The capacitor D is formed by theelectrode D1 composed of the part of the line portion L1 and theelectrode D2 (opposite electrode) connected to the ground terminal Gwhere the electrodes D1 and D2 face each other via an appropriatedielectric. The end of the line portion L2 other than the end connectedto the line portion L1 is open. On the other hand, the end of the lineportion L3 other than the end connected to the line portion L4 isconnected to the balanced terminal BT1, and the end of the line portionL4 other than the end connected to the line portion L3 is connected tothe balanced terminal BT2. Moreover, the connected parts of the lineportions L3 and L4 are connected to the ground terminal G.

Lengths of the above-mentioned line portions L1 to L4 vary depending onspecifications of the thin film balun 6. For example, the lengths may beset so as to form a quarter-wavelength (?14) resonator circuit of atransmission signal which is subject to conversion. Moreover, the lineportions L1 to L4 may be arbitrarily shaped so long as theabove-mentioned magnetic coupling is formed. Example shapes include aspiral (coil form), a zigzag, a straight line, and a curved line.

A basic operation of the thin film balun 6 is described below, withreference to FIG. 68. In the thin film balun 6, when an unbalancedsignal is input to the unbalanced terminal UT, the unbalanced signalpropagates through the line portions L1 and L2. By the magnetic coupling(first magnetic coupling) of the line portions L1 and L3 and themagnetic coupling (second magnetic coupling) of the line portions L2 andL4, the input unbalanced signal is converted to two balanced signalsthat differ in phase by 180° (π), and the two balanced signals areoutput from the balanced terminals BT1 and BT2. A converting operationfrom balanced signals to an unbalanced signal is the reverse of theabove-mentioned converting operation from an unbalanced signal tobalanced signals.

A wiring structure of the thin film balun is described below. FIG. 69 isa vertical sectional view schematically showing the wiring structure ofthe thin film balun 6. As shown in FIG. 69, the wiring layers M0, M1,M2, and M3 are formed in this order on the insulating substrate 100 ofalumina or the like. For instance, the unbalanced transmission line ULmentioned above is formed by the wiring layer M1, and the balancedtransmission line BL is formed by the wiring layer M2. The dielectriclayers 101 to 105 are formed between wires in the same wiring layer andbetween different wiring layers. For example, silicon nitride is used asthe dielectric layer 102 between the wiring layers M0 and M1 where thecapacitor D is formed, and the dielectric layer 104 between the wiringlayers M1 and M2 where the magnetic coupling between the unbalancedtransmission line UL and the balanced transmission line BL is formed.For example, alumina is used as the dielectric layers 101 and 103 inother parts. For example, polyimide is used as the dielectric layer 105.Note that the materials of these layers are not limited to the above,and not only inorganic insulators such as silicon nitride, alumina, andsilica but also organic insulators such as polyimide and epoxy resin maybe selected according to need. The unbalanced terminal UT, the balancedterminals BT1 and BT2, and the ground terminal G are formed through alldielectric layers. Thus, the thin film balun 6 is composed of a thinfilm multilayer structure formed on the insulating substrate 100.

A pattern of each of the wiring layers M0, M1, M2, and M3 of the thinfilm balun in the fourth embodiment is described in detail below. Coilportions are used as the line portions L1 to L4 in each of the followingexamples.

Example 1

FIGS. 70 to 73 are each a horizontal sectional view schematicallyshowing a different one of the wiring layers of the thin film balun 6 ofan example 1. As shown in FIGS. 70 to 73, the unbalanced terminal UT,the balanced terminals BT1 and BT2, and the ground terminal G are formedin all of the wiring layers M0 to M3. Each of the terminals UT, BT1,BT2, and G is electrically connected between different layers via athrough hole P. Note that all through holes P shown in FIGS. 70 to 73are plated with metal for electrical conduction of upper and lowerlayers. A structure of each wiring layer is described in detail below.

As shown in FIG. 70, the electrode D2 of the capacitor D is formed inthe wiring layer M0 on the insulating substrate 100, at a positionfacing a part of the coil portion C1 in the wiring layer M1. Theelectrode D2 is connected to the ground terminal G. The electrode D2 ofthe capacitor D is positioned so as to face the coil conductor 11 of thesecond line from the bottom of the coil portion C1 in plan view.

As shown in FIG. 71, the coil portion C1 (first coil portion, first lineportion) and the coil portion C2 (second coil portion, second lineportion) that constitute the unbalanced transmission line UL are formedadjacent to each other in the wiring layer M1. Each of the coil portionsC1 and C2 forms an equivalent of a quarter-wavelength (λ/4) resonator.The outer end 11 a of the coil conductor 11 constituting the coilportion C1 is connected to the unbalanced terminal UT, and the inner end11 b of the coil conductor 11 is connected to a through hole P. Theinner end 12 b of the coil conductor 12 constituting the coil portion C2is connected to a through hole P, and the outer end 12 a of the coilconductor 12 is open. A part of the coil conductor 11 in the outerperiphery of the coil portion C1 serves as the electrode D1 of thecapacitor D, and faces the electrode D2 in the wiring layer M0.Moreover, the coil conductor 12 constituting the coil portion C2 has alarger width than the coil conductor 11 constituting the coil portionC1, as shown in FIG. 71. As a result of conducting intense study, thepresent inventor has found that, in the case of inserting the capacitorD so that a part of the coil portion C1 serves as the electrode D1 ofthe capacitor D, excellent passage characteristics and balancecharacteristics can be obtained by forming the coil conductor 12 widerthan the coil conductor 11. Note, however, that there is no limitationon the widths and the numbers of turns of the coil conductors 12 and 11,and the widths and the numbers of turns of the coil conductors 12 and 11may be equal or different.

As shown in FIG. 72, the coil portion C3 (third coil portion, third lineportion) and the coil portion C4 (fourth coil portion, fourth lineportion) that constitute the balanced transmission line BL are formedadjacent to each other in the wiring layer M2. Each of the coil portionsC3 and C4 forms an equivalent of a quarter-wavelength (λ/4) resonator,as with the coil portions C1 and C2. The coil portions C3 and C4 of thebalanced transmission line BL are positioned facing the coil portions C1and C2 of the unbalanced transmission line UL respectively, and thefacing portions are magnetically coupled to form couplers. The outer end21 a of the coil conductor 21 constituting the coil portion C3 isconnected to the balanced terminal BT1, and the inner end 21 b of thecoil conductor 21 is connected to a through hole P. The outer end 22 aof the coil conductor 22 constituting the coil portion C4 is connectedto the balanced terminal BT2, and the inner end 22 b of the coilconductor 22 is connected to a through hole P. Moreover, the coilconductor 22 constituting the coil portion C4 has a larger width thanthe coil conductor 21 constituting the coil portion C3, as shown in FIG.72. As noted earlier, as a result of conducting intense study, thepresent inventor has found that, in the case of inserting the capacitorD so that a part of the coil portion C1 serves as the electrode D1 ofthe capacitor D, excellent passage characteristics and balancecharacteristics can be obtained by forming the coil conductor 22 widerthan the coil conductor 21. Note, however, that there is no limitationon the widths and the numbers of turns of the coil conductors 22 and 21,and the widths and the numbers of turns of the coil conductors 22 and 21may be equal or different.

As shown in FIG. 73, the wire 31 for connecting the coil portions C3 andC4 to the ground terminal G and the wire 32 for connecting the coilportions C1 and C2 are formed in the wiring layer M3. The wire 31 is abranch wire formed so as to connect two through holes P to the groundterminal G. The wire 31 is connected to the end 21 b of the coilconductor 21 and the end 22 b of the coil conductor 22 formed in thewiring layer M2, via the two through holes P. The wire 32 is connectedto the end 11 b of the coil conductor 11 and the end 12 b of the coilconductor 12 formed in the wiring layer M1, via through holes P.

Thus, in the example 1, the thin film balun 6 forming the equivalentcircuit shown in FIG. 68 is obtained by a multilayer wiring structure inwhich the two coil portions C1 and C2 constituting the unbalancedtransmission line are formed in the wiring layer M1 which is one layer,the two coil portions C3 and C4 constituting the balanced transmissionline are formed in the wiring layer M2 which is another layer adjacentto the wiring layer M1, the wire 32 connecting the coil portions C1 andC2 and the wire 31 connecting the coil portions C3 and C4 are formed inthe wiring layer M3 which is another layer adjacent to the wiring layerM2 on an opposite side to the wiring layer M1, and the electrode D2facing the part of the coil conductor 11 in the coil portion C1 to formthe capacitor D is formed in the wiring layer M0 which is another layeradjacent to the wiring layer M1 on an opposite side to the wiring layerM2.

Example 1A

FIG. 74 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 6A of an example 1A in the fourthembodiment. Structures other than the wiring layer M0 are the same asthose in the example 1. In the thin film balun 6A shown in FIG. 74, theelectrode D2 of the capacitor D is positioned so as to face the coilconductor 11 of the third line from the bottom of the coil portion C1 inplan view.

Example 1B

FIG. 75 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 6B of an example 1B. Structures other thanthe wiring layer M0 are the same as those in the example 1. In the thinfilm balun 6B shown in FIG. 75, the electrode D2 of the capacitor D inthe thin film balun 6A of the example 1A is shortened in length.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 6, 6A, and 6B of the examples described abovewere measured by simulation. Evaluation target frequencies (resonantfrequencies fr) of a transmission signal were set at 2400 MHz to 2500MHz. FIG. 76 is a diagram showing passage characteristic evaluationresults, FIG. 77 is a diagram showing phase difference evaluationresults, and FIG. 78 is a diagram showing amplitude differenceevaluation results. In each of FIGS. 76 to 78, curves E6, E6A, and E6Brespectively indicate evaluation results of the thin film baluns 6, 6A,and 6B. Meanwhile, a curve R indicates evaluation results of a thin filmbalun (comparative example) having the same structure except that thecapacitor D is not included.

The passage characteristics represent with how little loss a signalpasses through in an evaluation target frequency domain. 0 dB is idealpassage characteristics in the evaluation target frequency domain. Thepassage characteristics of each thin film balun were evaluated withdesired specifications being less than 1 dB in attenuation in theevaluation target frequency range. The phase difference is a differencein phase between two balanced signals output from the balanced terminalsBT1 and BT2, so that 180 deg is a more ideal phase balance. The phasebalance of each thin film balun was evaluated with desiredspecifications being from 170 deg to less than 190 deg in phasedifference in the evaluation target frequency range. The amplitudedifference is a difference in amplitude between two balanced signalsoutput from the balanced terminals BT1 and BT2, so that 0 dB is a moreideal output balance. The amplitude balance of each thin film balun wasevaluated with desired specifications being from −1 dB to 1 dB inamplitude difference in the evaluation target frequency range.

These results demonstrate that the thin film baluns 6, 6A, and 6B of theexamples maintain excellent characteristics in the passagecharacteristics and the phase balance. In particular, these resultsdemonstrate that the thin film baluns 6, 6A, and 6B exhibit excellentpassage characteristics as compared with the thin film balun R that doesnot include the capacitor, achieving an improvement of 0.13 dB at themaximum in passage characteristics relative to the comparative example.This value represents a significant improvement in passagecharacteristics while meeting the demand for smaller and thinner models.The results also demonstrate that the phase balance and the amplitudebalance can be adjusted by the position of the electrode D2 of thecapacitor D. In detail, it is demonstrated that the thin film balun 6 inwhich the electrode D2 faces a part of the coil conductor closer to theouter peripheral coil conductor than the inner peripheral coil conductorin the coil portion C1 exhibits most excellent passage characteristics,phase balance, and amplitude balance.

As noted earlier, the present invention is not limited to the aboveexamples, and various changes can be made without departing from thescope of the present invention. For example, the arrangement of theunbalanced terminal UT, the balanced terminals BT1 and BT2, and theground terminal G is not limited to the positions shown in the drawings.Moreover, the number of layers of the multilayer wiring structureforming the thin film balun may be smaller or larger than the numbershown in the drawings. Besides, the wiring layers on the insulatingsubstrate 100 may be reversed in order. Furthermore, various coilarrangements may be employed without departing from the scope of thepresent invention.

According to the thin film balun in the fourth embodiment of the presentinvention, a part of the first line portion that constitutes theunbalanced transmission line and is connected to the unbalanced terminalserves as one electrode of the capacitor, so that the thin film baluncan be made smaller and thinner while maintaining required baluncharacteristics. Such a thin film balun is widely applicable, inparticular to wireless communication devices, apparatuses, modules, andsystems which are required to be smaller and thinner, facilitiesincluding them, and manufacturing thereof.

Fifth Embodiment

A thin film balun in a fifth embodiment corresponding to the fifthaspect of the present invention is described below. FIG. 79 is anequivalent circuit diagram showing a structure of a thin film balun 7 inthe fifth embodiment of the present invention. The thin film balun 7includes the unbalanced transmission line (unbalanced circuit) UL inwhich the line portion L1 (first line portion) and the line portion L2(second line portion) are connected in series, and the balancedtransmission line (balanced circuit) BL in which the line portion L3(third line portion) and the line portion L4 (fourth line portion) areconnected in series. The line portions L1 and L3 form electromagneticcoupling, and the line portions L2 and L4 form electromagnetic coupling.

In the thin film balun 7, the end of the line portion L1 other than theend connected to the line portion L2 is connected to the unbalancedterminal UT, and the end of the line portion L2 other than the endconnected to the line portion L1 is an open end. On the other hand, theend of the line portion L3 other than the end connected to the lineportion L4 is connected to the balanced terminal BT1, and the end of theline portion L4 other than the end connected to the line portion L3 isconnected to the balanced terminal BT2. Moreover, the connected ends ofthe line portions L3 and L4 are connected to the ground terminal G.Furthermore, in this embodiment, a part of the line portion L4 of thebalanced transmission line BL is electrically connected to the groundterminal G via the capacitor D1.

Lengths of the above-mentioned line portions L1 to L4 vary depending onspecifications of the thin film balun 7. For example, the lengths may beset so as to form a quarter-wavelength (λ/4) resonator circuit of atransmission signal which is subject to conversion. Moreover, the lineportions L1 to L4 may be arbitrarily shaped so long as theabove-mentioned electromagnetic coupling is formed. Example shapesinclude a spiral (coil form), a zigzag, a straight line, and a curvedline.

A basic operation of the thin film balun 7 is described below, withreference to FIG. 79. When an unbalanced signal is input to theunbalanced terminal UT, the unbalanced signal propagates through theline portions L1 and L2. By the electromagnetic coupling (firstelectromagnetic coupling) of the line portions L1 and L3 and theelectromagnetic coupling (second electromagnetic coupling) of the lineportions L2 and L4, the input unbalanced signal is converted to twobalanced signals that have the same frequency as the unbalanced signaland differ in phase by 180° (π), and the two balanced signals are outputfrom the balanced terminals BT1 and BT2. On the other hand, when twobalanced signals of the same magnitude that have a predeterminedfrequency and differ in phase by 180° are input to the balancedterminals BT1 and BT2, an unbalanced signal of the same frequency as thebalanced signals is output from the unbalanced terminal UT.

A wiring structure of the thin film balun in the fifth embodiment isdescribed below. FIG. 80 is a vertical sectional view schematicallyshowing the wiring structure of the thin film balun 7. As shown in FIG.80, the wiring layers M0, M1, and M2 are formed in this order on theinsulating substrate 100 of alumina or the like. For instance, thecapacitor electrode DE1 and the connection wires (connectors) connectingthe line portions in the balanced transmission line BL and theunbalanced transmission line UL are formed by the wiring layer M0, thebalanced transmission line BL is formed by the wiring layer M1, and theunbalanced transmission line UL is formed by the wiring layer M2. Thedielectric layers 101 to 105 are formed between wires in the same wiringlayer and between different wiring layers. For example, silicon nitrideis used as the dielectric layer 102 between the wiring layers M0 and M1where the capacitor D1 is formed, and the dielectric layer 104 betweenthe wiring layers M1 and M2 where the electromagnetic coupling betweenthe unbalanced transmission line UL and the balanced transmission lineBL is formed. For example, alumina is used as the dielectric layers 101and 103 covering the wiring layers M0 and M1. For example, polyimide isused as the dielectric layer 105 covering the wiring layer M2. Note thatthe materials of these layers are not limited to the above, and not onlyinorganic insulators such as silicon nitride, alumina, and silica butalso organic insulators such as polyimide and epoxy resin may beselected according to need. The unbalanced terminal UT, the balancedterminals BT1 and BT2, and the ground terminal G are formed through alldielectric layers. Thus, the thin film balun 7 is composed of a thinfilm multilayer structure formed on the insulating substrate 100.

As shown in FIG. 80, in the fifth embodiment, the above-mentionedcapacitor D1 is formed between the wiring layer M1 that includes thebalanced transmission line BL and the wiring layer M0 that faces thewiring layer M1 via the dielectric layer 102 and includes the capacitorelectrode DEL

A pattern of each of the wiring layers M0, M1, and M2 of the thin filmbalun in the fifth embodiment is described in detail below. Coilportions are used as the line portions L1 to L4 in each of the followingexamples.

Example 1A

FIGS. 81 to 83 are each a horizontal sectional view schematicallyshowing a different one of the wiring layers of a thin film balun 7A ofan example 1A in the fifth embodiment. As shown in FIGS. 81 to 83, theunbalanced terminal UT, the balanced terminals BT1 and BT2, and theground terminal G are formed in all of the wiring layers M0 to M2. Eachof the terminals UT, BT1, BT2, and G is electrically connected betweendifferent layers via a through hole P. Note that all through holes Pshown in FIGS. 81 to 83 are plated with a metal conductor for electricalconduction of upper and lower layers. A structure of each wiring layeris described in detail below.

As shown in FIG. 81, the wire 31 for connecting the coil portions C3 andC4 of the balanced transmission line BL to the ground terminal G and thewire 32 for connecting the coil portions C1 and C2 of the unbalancedtransmission line UL are formed in the wiring layer M0 on the insulatingsubstrate 100. The wire 31 is a branch wire formed so as to connect twothrough holes P to the ground terminal G. In addition, the capacitorelectrode DE1 of the capacitor D1 is formed in the wiring layer M0 at aposition facing a part of the coil portion C4 in the wiring layer M1.The capacitor electrode DE1 is connected to the ground terminal G. Inmore detail, the capacitor electrode DE1 of the capacitor D1 ispositioned extending vertically so as to face the coil conductor of thesecond line and one half of the coil conductor of the first line fromthe right of the coil portion C4 in plan view.

As shown in FIG. 82, the coil portion C3 (third line portion) and thecoil portion C4 (fourth line portion) that constitute the balancedtransmission line BL are formed adjacent to each other in the wiringlayer M1. Each of the coil portions C3 and C4 forms an equivalent of aquarter-wavelength (λ/4) resonator. The outer end 21 a of the coilconductor 21 constituting the coil portion C3 is connected to thebalanced terminal BT1, and the inner end 21 b of the coil conductor 21is connected to a through hole P. The outer end 22 a of the coilconductor 22 constituting the coil portion C4 is connected to thebalanced terminal BT2, and the inner end 22 b of the coil conductor 22is connected to a through hole P. The coil conductors 21 and 22 areconnected to each other and also connected to the ground terminal G, viathe wire 31 in the wiring layer M0. A part of the coil conductor 22 ofthe coil portion C4 serves as a capacitor electrode of the capacitor D1,and faces the capacitor electrode DE1 in the wiring layer M0. Note thatthere is no limitation on the widths and the numbers of turns of thecoil conductors 22 and 21, and the widths and the numbers of turns ofthe coil conductors 22 and 21 may be equal or different.

As shown in FIG. 83, the coil portion C1 (first line portion) and thecoil portion C2 (second line portion) that constitute the unbalancedtransmission line UL are formed adjacent to each other in the wiringlayer M2. Each of the coil portions C1 and C2 forms an equivalent of aquarter-wavelength (λ/4) resonator. The coil portions C1 and C2 of theunbalanced transmission line UL are positioned facing the coil portionsC3 and C4 of the balanced transmission line BL respectively, and thefacing portions are electromagnetically coupled to form couplers. Theouter end 11 a of the coil conductor 11 constituting the coil portion C1is connected to the unbalanced terminal UT, and the inner end 11 b ofthe coil conductor 11 is connected to a through hole P. The inner end 12b of the coil conductor 12 constituting the coil portion C2 is connectedto a through hole P, and the outer end 12 a (open end) of the coilconductor 12 is open. The coil conductors 11 and 12 are connected toeach other via the wire 32 in the wiring layer M0. Note that there is nolimitation on the widths and the numbers of turns of the coil conductors12 and 11, and the widths and the numbers of turns of the coilconductors 12 and 11 may be equal or different.

Thus, in the example 1A, the thin film balun 7A forming the equivalentcircuit shown in FIG. 79 is obtained by a multilayer wiring structure inwhich the two coil portions C3 and C4 constituting the balancedtransmission line BL are formed in the wiring layer M1 which is onelayer, the two coil portions C1 and C2 constituting the unbalancedtransmission line UL are formed in the wiring layer M2 which is anotherlayer adjacent to the wiring layer M1, and the wire 32 connecting thecoil portions C1 and C2, the wire 31 connecting the coil portions C3 andC4, and the capacitor electrode DE1 facing the part of the coilconductor 22 in the coil portion C4 to form the capacitor D1 are formedin the wiring layer M0 which is another layer adjacent to the wiringlayer M1 on an opposite side to the wiring layer M2.

Example 1B

FIG. 84 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 7B of an example 1B. Structures other thanthe wiring layer M0 are the same as those in the example 1A. In the thinfilm balun 7B shown in FIG. 84, the capacitor electrode DE1 ispositioned extending vertically so as to face the coil conductor 22 ofthe second line and one half of the coil conductor 22 of the third linefrom the right of the coil portion C4 in plan view.

Comparative Example 1R

FIG. 85 is an equivalent circuit diagram showing a structure of a thinfilm balun 7R of a comparative example 1R. FIGS. 86 and 87 arehorizontal sectional views schematically showing the wiring layers M0and M1 of the thin film balun 7R of the comparative example 1R,respectively. Structures other than the wiring layers M0 and M1 are thesame as those in the example 1A.

As shown in FIG. 85, in the thin film balun 7R of the comparativeexample, the capacitor D1 is connected between the ground terminal G andthe balanced terminal BT2 in a form of branching from the line portionL4 independently. As shown in FIG. 86, a capacitor electrode DE11connected to the ground terminal G is provided in the wiring layer M0 soas to face a region outside the coil portion C4 in the wiring layer M1.As shown in FIG. 87, a capacitor electrode DE12 facing the capacitorelectrode DE11 is formed in the wiring layer M1. The capacitor electrodeDE12 is provided independently of the coil portion C4 of the balancedtransmission line BL, and has one end connected to the balanced terminalBT2.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 7A, 7B, and 7R described above were measured bysimulation. Evaluation target frequencies (resonant frequencies fr) of atransmission signal were set at 2400 MHz to 2500 MHz. FIG. 88 is adiagram showing passage characteristic evaluation results, FIG. 89 is adiagram showing phase difference evaluation results, and FIG. 90 is adiagram showing amplitude difference evaluation results. In each ofFIGS. 88 to 90, curves E7A, E7B, and E7R respectively indicateevaluation results of the thin film baluns 7A, 7B, and 7R.

The passage characteristics represent with how little loss a signalpasses through in an evaluation target frequency domain. 0 dB is idealpassage characteristics in the evaluation target frequency domain. Thephase difference is a difference in phase between two balanced signalsoutput from the balanced terminals BT1 and BT2, so that 180 deg is amore ideal phase balance. The amplitude difference is a difference inamplitude between two balanced signals output from the balancedterminals BT1 and BT2, so that 0 dB is a more ideal output balance.

These results demonstrate that the thin film baluns 7A and 7B of theexamples maintain excellent characteristics in both the passagecharacteristics and the balance characteristics, as compared with thethin film balun 7R of the comparative example. Moreover, the results ofthe examples 1A and 1B demonstrate that, by adjusting the position ofthe capacitor D1, the balance characteristics can be adjusted whilemaintaining excellent passage characteristics.

Example 1C

FIG. 91 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 7C of an example 1C according to thepresent invention. Structures other than the wiring layer M0 are thesame as those in the example 1A. In the thin film balun 7C shown in FIG.91, the capacitor electrode DE1 is positioned extending horizontally soas to face the coil conductor 22 of the second line from the bottom ofthe coil portion C4 in plan view.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of thethin film balun 7C described above were measured by simulation.Evaluation target frequencies (resonant frequencies fr) of atransmission signal were set at 2400 MHz to 2500 MHz. FIG. 92 is adiagram showing passage characteristic evaluation results, FIG. 93 is adiagram showing phase difference evaluation results, and FIG. 94 is adiagram showing amplitude difference evaluation results. In each ofFIGS. 92 to 94, curves E7C and E7R respectively indicate evaluationresults of the thin film baluns 7C and 7R.

These results demonstrate that the thin film balun 7C of the example 1Cmaintains excellent characteristics in both the passage characteristicsand the balance characteristics, as compared with the thin film balun 7Rof the comparative example 1R.

Example 1D

FIG. 95 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 7D of an example 1 D. Structures otherthan the wiring layer M0 are the same as those in the example 1A. In thethin film balun 7D shown in FIG. 95, the capacitor electrode DE1 ispositioned extending horizontally so as to face the coil conductor 22 ofthe second line from the top of the coil portion C4 in plan view.

Example 1E

FIG. 96 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 7E of an example 1E. Structures other thanthe wiring layer M0 are the same as those in the example 1A. In the thinfilm balun 7E shown in FIG. 96, the capacitor electrode DE1 ispositioned extending horizontally and vertically so as to face the coilconductor 22 of the second line from the top of the coil portion C4 andthe coil conductor 22 of the second line from the right of the coilportion C4 in plan view.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 7D and 7E described above were measured bysimulation. Evaluation target frequencies (resonant frequencies fr) of atransmission signal were set at 2400 MHz to 2500 MHz. FIG. 97 is adiagram showing passage characteristic evaluation results, FIG. 98 is adiagram showing phase difference evaluation results, and FIG. 99 is adiagram showing amplitude difference evaluation results. In each ofFIGS. 97 to 99, curves E7D, E7E, and E7R respectively indicateevaluation results of the thin film baluns 7D, 7E, and 7R.

These results demonstrate that the thin film baluns of the examples 1Dand 1E according to the present invention maintain excellentcharacteristics in both the passage characteristics and the balancecharacteristics, as compared with the thin film balun of the comparativeexample 1R.

Example 2

FIG. 100 is an equivalent circuit diagram showing a structure of a thinfilm balun of an example 2 in the fifth embodiment. In a thin film balun8 of the example 2, the capacitor D2 is introduced between the groundterminal G and the line portion L3 of the balanced transmission line BL.

Example 2A

A pattern of the wiring layer M0 in the example of the thin film balunhaving the circuit structure shown in FIG. 100 is described in detailbelow. FIG. 101 is a horizontal sectional view schematically showing thewiring layer M0 of a thin film balun 8A of an example 2A. Structuresother than the wiring layer M0 are the same as those in the example 1A.In the thin film balun 8A shown in FIG. 101, the capacitor electrode DE2is positioned extending horizontally so as to face the coil conductor 21of the second line from the bottom of the coil portion C3 of thebalanced transmission line BL in plan view.

Example 2B

FIG. 102 is a horizontal sectional view schematically showing the wiringlayer M0 of a thin film balun 8B of an example 2B. Structures other thanthe wiring layer M0 are the same as those in the example 1A. In the thinfilm balun 8B shown in FIG. 102, the capacitor electrode DE2 ispositioned extending horizontally so as to face the coil conductor 21 ofthe second line from the top of the coil portion C3 in plan view.

Comparative Example 2R

FIG. 103 is an equivalent circuit diagram showing a structure of a thinfilm balun 8R of a comparative example FIGS. 104 and 105 are horizontalsectional views schematically showing the wiring layers M0 and M1 of thethin film balun 8R of the comparative example, respectively. Structuresother than the wiring layers M0 and M1 are the same as those in theexample 1A.

As shown in FIG. 103, in the thin film balun 8R of the comparativeexample, the capacitor D2 is connected between the ground terminal G andthe balanced terminal BT1 in a form of branching from the line portionL3 independently. As shown in FIG. 104, a capacitor electrode DE21connected to the ground terminal G is provided in the wiring layer M0 soas to face a region outside the coil portion C3 in the wiring layer M1.As shown in FIG. 105, a capacitor electrode DE22 facing the capacitorelectrode DE21 is formed in the wiring layer M1. The capacitor electrodeDE22 is provided independently of the coil portion C3 of the balancedtransmission line BL, and has one end connected to the balanced terminalBT1.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of eachof the thin film baluns 8A, 8B, and 8R described above were measured bysimulation. Evaluation target frequencies (resonant frequencies fr) of atransmission signal were set at 2400 MHz to 2500 MHz. FIG. 106 is adiagram showing passage characteristic evaluation results, FIG. 107 is adiagram showing phase difference evaluation results, and FIG. 108 is adiagram showing amplitude difference evaluation results. In each ofFIGS. 106 to 108, curves E8A, E8B, and E8R respectively indicateevaluation results of the thin film baluns 8A, 8B, and 8R.

These results demonstrate that the thin film baluns 8A and 8B of theexamples maintain excellent characteristics in both the passagecharacteristics and the balance characteristics, as compared with thethin film balun 8R of the comparative example.

Example 3

FIG. 109 is an equivalent circuit diagram showing a structure of a thinfilm balun of an example 3 in the fifth embodiment of the presentinvention. In a thin film balun 9 of the example 3, not only thecapacitor D1 is introduced between the ground terminal G and the lineportion L4 of the balanced transmission line BL, but also the capacitorD2 is introduced between the ground terminal G and the line portion L3.

Example 3A

A pattern of the wiring layer M0 in the example of the thin film balunhaving the circuit structure shown in FIG. 109 is described in detailbelow. FIG. 110 is a horizontal sectional view schematically showing thewiring layer M0 of a thin film balun 9A of an example 3A according tothe present invention. Structures other than the wiring layer M0 are thesame as those in the example 1A. In the thin film balun 9A shown in FIG.110, the two capacitor electrodes DE1 and DE2 connected to the groundterminal G are formed in the wiring layer M0. The capacitor electrodeDE1 is positioned extending vertically so as to face the coil conductor22 of the second line from the right of the coil portion C4 in planview. The capacitor electrode DE2 is positioned extending horizontallyso as to face the coil conductor 21 of the second line from the bottomof the coil portion C3 in plan view.

(Characteristic Evaluation)

Passage characteristics (insertion loss) and balance characteristics(phase difference and amplitude difference of balanced signals) of thethin film balun 9A described above were measured by simulation.Evaluation target frequencies (resonant frequencies fr) of atransmission signal were set at 2400 MHz to 2500 MHz. FIG. 111 is adiagram showing passage characteristic evaluation results, FIG. 112 is adiagram showing phase difference evaluation results, and FIG. 113 is adiagram showing amplitude difference evaluation results. In each ofFIGS. 111 to 113, a curve E9A indicates evaluation results of the thinfilm balun 9A. Meanwhile, curves E7R and E8R in each of FIGS. 111 to 113respectively indicate evaluation results of the thin film baluns 7R and8R of the comparative examples where the capacitor is providedindependently.

These results demonstrate that the thin film balun 9A of the exampleexhibits excellent characteristics in both the passage characteristicsand the balance characteristics, as compared with the thin film baluns7R and 8R of the comparative examples.

As noted earlier, the present invention is not limited to the aboveexamples, and various changes can be made without departing from thescope of the present invention. For example, though the part of the coilportion C3 or C4 constituting the balanced transmission line BL servesas a capacitor electrode in the examples, a layer of another capacitorelectrode connected to the coil portion C3 or C4 may be formed betweenthe coil portion C3 or C4 and the capacitor electrode DE2 or DE1.Moreover, the arrangement of the unbalanced terminal UT, the balancedterminals BT1 and BT2, and the ground terminal G is not limited to thepositions shown in the drawings. Besides, the wiring layers on theinsulating substrate 100 may be reversed in order. Furthermore, variouscoil arrangements may be employed without departing from the scope ofthe present invention.

According to the thin film balun in the fifth embodiment of the presentinvention, the capacitor is introduced between the ground terminal and apart of the balanced transmission line, so that the thin film balun canbe made smaller and thinner while maintaining required baluncharacteristics. Such a thin film balun is widely applicable, inparticular to wireless communication devices, apparatuses, modules, andsystems which are required to be smaller and thinner, facilitiesincluding them, and manufacturing thereof.

What is claimed is:
 1. A thin film balun comprising: an unbalancedtransmission line; a balanced transmission line facing the unbalancedtransmission line and electromagnetically coupled to the unbalancedtransmission line; a capacitor electrode facing a part of the unbalancedtransmission line that faces the balanced transmission line, to form acapacitor; and an unbalanced terminal connected to the unbalancedtransmission line and the capacitor electrode.
 2. The thin film balunaccording to claim 1, wherein the part of the unbalanced transmissionline serves as an opposite electrode to form the capacitor together withthe capacitor electrode.
 3. The thin film balun according to claim 1,wherein the unbalanced transmission line is provided in a first layer,the balanced transmission line is provided in a second layer, thecapacitor electrode is provided in a third layer, the balancedtransmission line is facing the unbalanced transmission line in alamination direction, and the capacitor electrode is facing the part ofthe unbalanced transmission line in the lamination direction.
 4. A thinfilm balun comprising an unbalanced transmission line including a firstline portion and a second line portion; a balanced transmission lineincluding a third line portion and a fourth line portion that arepositioned facing the first line portion and the second line portion andelectromagnetically coupled to the first line portion and the secondline portion, respectively; a first capacitor electrode facing a part ofthe second line portion that faces the fourth line portion, to form afirst capacitor; and an unbalanced terminal connected to the first lineportion and the first capacitor electrode.
 5. The thin film balunaccording to claim 4, further comprising: a second capacitor electrodefacing a part of the first line portion that faces the third lineportion, to form a second capacitor, wherein the unbalanced terminal isconnected to the first line portion, the first capacitor electrode andthe second capacitor electrode.
 6. The thin film balun according toclaim 5, wherein the first and second capacitor electrodes are formed ina first wiring layer the first line portion and the second line portionare formed in a second wiring layer, and the third line portion and thefourth line portion are formed in a third wiring layer.
 7. The thin filmbalun according to claim 6, wherein one end of the first line portion isconnected an unbalanced terminal, the other end of the first lineportion is connected to one end of the second line portion through thefirst wiring formed on a fourth wiring layer, the other end of thesecond line portion is an open end, one end of the third line portion isconnected to a first balanced terminal, the other end of the third lineportion is connected to one end of the fourth line portion through thefirst wiring formed on the fourth wiring layer, and the other end of thefourth line portion is connected to a second balanced terminal.
 8. Thethin film balun according to claim 7, wherein the second wiring isconnected to a ground terminal through a third wiring formed in thefourth wiring layer.
 9. The thin film balun according to claim 4,wherein the first line portion is longer than the third line portion.10. The thin film balun according to claim 4, wherein the unbalancedterminal is disposed closer to the first line portion than the secondline portion.
 11. A thin film balun comprising: an unbalancedtransmission line including a first line portion and a second lineportion; a balanced transmission line including a third line portion anda fourth line portion that are positioned facing the first line portionand the second line portion and magnetically coupled to the first lineportion and the second line portion, respectively; an unbalancedterminal connected to the first line portion; a group terminal connectedto the first line portion via a C component; and an opposite electrodeconnected to the ground terminal and facing a part of the first lineportion, wherein the second line portion has a larger width than thefirst line portion, the fourth line portion has a larger width than thethird line portion, and the C component is formed by the oppositeelectrode and the part of the first line portion.
 12. The thin filmbalun according to claim 11, wherein each of the second line portion andthe fourth line portion has a larger width than the first line portionand the third line portion.
 13. The thin film balun according to claim11, wherein the first line portion includes a first coil portion, thesecond lone portion includes a second coil portion, the third lineportion include a third coil portion, and the fourth line portionincludes a fourth coil portion.
 14. The thin film balun according toclaim 13, wherein the opposite electrode faces a part of a coilconductor that is closer to an outer peripheral coil conductor than aninner peripheral coil conductor in the first coil portion.
 15. The thinfilm balun according to claim 11, wherein the opposite electrode isformed in a first wiring layer, the first line portion and the secondline portion are formed in a second wiring layer, and the third lineportion and the fourth line portion are formed in a third wiring layer.16. The thin film balun according to claim 15, wherein one end of thefirst line portion is connected to the unbalanced terminal, the otherend of the first line portion is connected to one end of the second lineportion through the first wiring formed on a fourth wiring layer, theother end of the second line portion is an open end, one end of thethird line portion is connected to a first balanced terminal, the otherend of the third line portion is connected to one end of the fourth lineportion through the first wiring formed on the fourth wiring layer, andthe other end of the fourth line portion is connected to a secondbalanced terminal.
 17. The thin film balun according to claim 16,wherein the second wiring is connected to a ground terminal through athird wiring formed in the fourth wiring layer.
 18. The thin film balunaccording to claim 11, wherein the first line portion is longer than thethird line portion, and the second line portion is longer than thefourth line portion.